First off you don't need to go and buy anymore kites, you have a capable bunch there which you can trick with.
You clearly have what it takes to progress in this hobby, the first being patience alongside
persistence and the second being not afraid to ask for advice. These attributes will serve you well
I'm not that good at providing concise answers to questions so please prepare yourself for a bout of typing diarrhoea. Most fliers find it difficult to explain exactly what they are doing to get a trick to work as it just becomes part of 'muscle memory' but I'll try to give you some pointers.
I feel the snap stall is often overlooked as being a pretty tough thing to get right. When you have been flying a while it is easy to see it merely as a way to get into another trick, or a way to stop the kite whereever in the window you choose, as opposed to a trick in its own right.
As Kevin has already stated often the answer to most tricks success lies in the skill of using your feet correctly. This can feel odd at first as the three things we are more likely to be thinking about when flying are a) watching the kite, b) using your hands and c) not crashing.
Let's consider how a 'normal' kite flies. The wind hits the sail and provides forward drive due to you holding the other end of the lines. That is what allows us to fly the kite around the sky. Now trick flying has an extra element to this, we need to remove that forward drive by stalling it in order to perform a trick. We therefore need to dump the wind out of the sail. Now this loss of air can only be a temporary thing. Sooner or later the wind is going to get the better of you and repower the kite and thus send it off flying again. All we can try and do is prolong
this latter part from happening.
Now lets break the snap stall down into 3 elements. The first is the hand movement to dump the air, the second being prolonging this dump, and the third is reading when you can't hold it any longer. We'll forget the latter for now as this becomes more relevant when you are using the snap stall to get into another trick. So we are left with the first two. From reading your post I think you have the first part more or less sorted. You can get the kite to stop. The problem lies in prolonging this 'stoppage'. Therefore we need to use our feet and move/lunge/run forward towards the kite. If you are already doing this then you need to be doing it more
Trick flying is sometimes called slack line flying, I find this a much better explanation to describe what we are doing. Rather than taut lines we need slack lines to allow us to trick (ok this is not totally applicable to all tricks). Yes, quick wrists will give us some slack but coupled with feet we will then get more
slack. If you watch a good flyer they very rarely are standing in one place. There is a constant ebb and flow of moving backwards and forwards.This comes from a feel for what the wind is doing and practice will achieve this.
So the more you can walk/run into the kite the longer that snap stall is going to hold. 8-10mph wind is what most fliers enjoy. Any more and we have to do more leg work and above 15mph it can become nigh on impossible to get enough slack into the lines.
Here are a few tips that may help you. Please bear with me on the first one, it's not perfectly accurate but hopefully you'll get the point lol.
1. Imagine you are in an orchestra and you are the percussionist. You have two kettle drums in front of you, one on each side. We are holding the sticks in front of us waiting for the nod from the conductor to do a 'boom, boom'; one boom on each drum. We get the nod and we quickly hit each drum one after another. This wants to be a 'boom, boom' as in what you may hear at the end of a joke, but quicker. As soon as we have done it we then raise the sticks to await the next nod from the conductor. This is more or less the hand movements we need to snap the kite. If done right you are going to hear the kite make a particular noise, a bit like 'chick, chick'.
2. Sometimes I think I am walking/running forward enough but maybe i'm not. A great tip my flying buddy did with me was to lay something on the ground right next to my feet. He then would make me do the trick and then stop moving as soon as the kite flew off. Without looking around he would ask me to guess how far I had moved forward. I was very rarely right. I always thought I had moved more forward than what I actually had. After a while he made me repeat the process but he would lay another marker on the floor or where he felt I should be running to. This really opened my eyes as to how much slack can sometimes be required. I've seen people sprint many, many metres just for one trick.
3. Try and video yourself either from behind or from the side. It doesn't matter how ugly the flying looks. That is not the purpose. The benefit comes from watching it back and you may realise that what you thought you were doing was actually nothing like what you were really doing. I learnt a lot from doing this.
4. Try walking into the kite as you are doing the wrist movements (and then continue moving forwards) rather than after.
5. Flying on your own can be very lonely and soul destroying if you are struggling with something. This is often what leads to people moving onto different hobbies. Flying with others will bring you on leaps and bounds, and you will learn way more from watching the flier than the kite.
Now I see that you live in Chesterfield. There are a bunch of us (NMKG) who regularly fly and are not far from you, all based around Leeds way, so you are about an hours drive away. Why don't you meet up with us? I am confident that at the end of the day you would be driving back with a smile on your face. Failing that we could meet half way, travel down your way or come and join us at Filey in September. You can click on the link in my signature to find out more or visit our website here
(please note this is a work in progress and has a lot of work needed before it's finished, life just seems to be getting in the way at the minute).
Don't get too hung up on a particular trick. Sometimes you just need to put it down for a while and try some other move. You can still trick without being perfect at snap stalls. We always tend to teach people how to axel first as its pretty easy to get your head around and is a great confidence booster.
Have you spotted the Tricky Wiki link at the top of this forum? It is a list of tricks and advice on how to achieve them.
Whatever you choose to do, do not give up. You'll get there