I'm painting a portrait of a person in acrylics and I'm having a great problem in painting the skin of the face. For some reason the colours don't seem to be able to remain on the canvas. It kind of gets wiped off with the next brush stroke. I don't know why this is happening and what I can do to solve it. Any ideas?

from what your saying i believe the acrylics you are using may be to thinned out for the surface your painting on. try using some heavy body acrylics, those are thick acrylics.