Luvs2ride, I agree the pad can be a big culprit. If the saddle does in fact fit (but just barely), a thick pad could make it too narrow. Or, from my own experience with the rolley - polley body types, even when the saddle fits with a thicker pad, it still seems to slip more than when I use a thinner, more pliable pad.
But as others mentioned, the actual saddle fit is the first thing you need to evaluate.
A mounting block may also help you out, as you don't need to put so much weight in the stirrup to get up there.