I've heard that slippage happens when the saddle doesn't fit well but this saddle appears to fit her pretty well. She's also the type that acts out when the saddle isn't any good and she's doing mostly fine (she has gotten more antsy about being mounted as of late but she's not rearing or taking off as soon as I get on and she's never been good at standing still for mounting). She has been nickering significantly when I get off but she's always done that a bit now and then, now it's just every time. I think it might have something to do with the pad being smooshed into her withers (she has pretty high withers for an Arab) because it shifts down as the saddle slips around.
Here's a picture (it's just a random one my friend took so I'm pretty sure it wasn't cinched up yet and the back strap is under the saddle) from a few weeks ago, tomorrow I will update with better pictures if need be.