Haunted saddle.. (20 characters)?

So I have this saddle and it always slips sideways, like I will put it real tight and I have a brest plate it fits good ive had 2 professional saddle fitters out and when ever my friends ride it's fine but if me or my sister ride in it it flips under the 2 of my horses it fits and we fall I've never had this problem and I have fairly good balance, it's western btw, It will go right under my horse. And I make sure she sucks in before I tighten it, what could be the problem?!