Saddlepad moves forwards - saddle stays put

[milliepops]

Little cob has learnt to move through her back which is fabulous and gives me a super feeling.
When she lets go, she feels like a gigantic springy warmblood despite being a 14hh section D

She's currently wearing Millie's saddle with a Mattes half pad underneath which has shims to adjust the fit. It appears to fit her well with this, and she certainly seems very comfortable - she's the sort that will jack it in if she's not happy.
On a schooling day, after a few mins of trotting & cantering the pad start to move forward.

Saddle doesn't budge, nor does the saddlecloth underneath it. She has a fairfax girth which fits well. It's only started to happen since she's found out how to really move.

Any thoughts, beyond 'sew some better tabs on the half pad'?
Never come across this before (physio coming tomorrow as a routine thing so will ask her too)

 

[Roody2]

What order does the saddle pad/mattes pad go on?

Is the mattes pad sheepskin lined? If yes and it's directly on her back you could try using a plain pad to put the shims in.

Only othe thought is whether the girth is done up enough.

 

[Horsemad12]

Strange it is moving forward, how is it shimmed?