Have you tried putting the diaper either looser or tighter? If it's too tight then that can cause irritation and similarly if it's too loose (being too loose may cause it to shift and move more).

Have you tried another diaper, a Sandy's perhaps? Maybe it's just the binding on the One-Size that isn't working all that great and another style of diaper would work better?

Have you showed the rash to a doctor to see what he/she thinks? Sometimes persistant rashes can be something like yeast and once treated with anti-fungals disappear completly. My dd had a nasty rash that would come/go for months, I would treat the heck out it with all kinds of methods, including disposables for days, even weeks. The rash would go away, I'd put her back in cloth and the rash would come back. Finally had a doc look at it, she gave us a prescription for an anti-fungal & cortisone cream and it disappeared completly. Some also just use over-the-counter creams for female yeast infections, they're apparently pretty much the same thing.

Karen.