When was the last time you checked your saddle fit? If it has been a while then you might want to check your fit. If that is your problem then you might want to change your gullet size or mabe you might want a saddle fitter to fit your horse with a new one. This may have happened because your horse is young and they are still growing or mabe they're getting a lot more work than they used to.
Have you though mabe it's your saddle pad? It could be slippery and the saddle falls back because of it. If so then you could invest in a Non-Slip Pad. This can also help with a slippery back mabe the shampoo works too good?
Have you thought it might be what your doing? Mabe it's just that your jumping really high and your saddle slips back because their back goes almost vertical. If that's it mabe you should invest in an breastplate I prefer one like this Pessoa Saddles myself. This will keep your saddle from slipping back if it is fitted properly. I also segest if you are jumping super high to use a crupper like this Crupper - standard folder leather tail piece - lavender/violet beta I only use one when jumping super high I don't really like them but don't be afraid to use one if your saddle slips.
If this didn't help try to think..
When dose it slip?
Have I looked to see if my saddle fits, that I don't need a breastplate and my saddle pad isn't sliping?
...and tell me and I might be able to help.