A slip pad. It's a little rubber square that you put between your saddle and the bounce pad. So it stays in place.
I've heard on the fairly new kavaliere ( idk if that's how you spell it) and some people have them and I've heard some good reviews!
Grippy half pad or at least the ones that aren't shiny
You can get thin gel grippy half pads that might help! I know presidents choice makes something that goes under the saddle pad itself but that might just be for saddle pads slipping, maybe pair that with a gel pad to stop both if you think that might help? Ecogold makes some really nice (but really expensive) half pads that can stop your saddle from slipping!
Mine slips back too I just got a breastplate. Mine horse has a high withers.
Some kind of pad if not breastplate