slipping saddle pad

My friend just bought a brand new Tucker saddle that was custom made to fit her horse. The saddle fits her horse well, but it seems like every pad she tries slips right out from under the saddle. I have let her try my Supracor pad without the cover and my Supracor with the suede cover. Both of those slipped out also. Her rounded pad slipped out and the square professional choice that she just bought also slips out. She said that the same thing happened with her last saddle also on this horse. She thinks it might be she shape of her horse or something.

Has anyone else had this problem? What might be done to fix it?

I've had this problem too. What I did was I bought a rather roughed out sweat blanket and put it under a really well broken in fleece backed pad. But, something else you could try is to buy a saddle pad with a neoprene backing. Those don't require sweat blankets and they don't slip because all the rubber bumps keep it in place. The one I'm most familiar with is the "Tacky Two" pad made by Reinsman.

Some tack stores have no slip grip pads, which go inbetween the saddle pad and the saddle. It grips to both and keeps them in place