[MPWR]

Acrylics seem to stick just fine to Tamiya primer- but I've never used Humbrol enamels, so this could be the problem. Is the paint peeling down to the layer of white primer, or to bare plastic? If it's just to the white primer, and the primer appears more or less intact, you can strip the rest of the paint while not harming the plastic by soaking in CSC/Purple Power or the like. Give it a good scrubbing with dish soap and an old toothbrush, and paint it again- with something else!