A resi and a crash mat are not the same thing. A Resi pit is large soft mat which is about 80 cms high, with a soft top to make landing on it like landing in a foam pit. You don't see them very much in Australia.
If you have a good rebound but then you go down in your back tuck then that usually means you are rushing the back tuck, you are probably tucking over into your back salto before you have rebounded to your full height out of your flic. You need to allow you rebound to reach full height and then complete the tuck.
Good drills are to use a large box, a mini tramp and a crash mat. Do a handstand on the large box and corbut down onto the mini tramp and practice waiting long to do the back salto onto the crash mat. The mini tramp will give you extra height and the crash mat a soft enough landing that it will help you feel confident enough to wait long before completing the tuck.