I am painting a wall that has flat paint. Part of the wall paint is peeling off as I paint with the roller. The wall had no peeling paint when I started, now the paint is peeling onto my roller as I paint.
A possible cause is a layer of cheap flat paint. All that you can do is carefully apply a primer before the paint. Your only other choice is to try to remove some of problem paint then prime. One way to remove the problem paint is with plain warm water and a sponge. It might be that easy.
Once it is primed you shouldn't have anymore problems.