I have generally not had too much problem painting PE or resin with enamels. I've had some minor problems using acrylics, but I think that was largely due to thinning & air brush pressure issues. Surface preparation is the key.
You may be experiencing problems on the PE associated with grease & oil. Skin oils are troublesome if you have been fingering the parts. Clean the PE fret by dumping it in some lacquer thinner. Swish & allow to air dry.
Oils & release agent can be troublesome on resin too. Wash the parts in warm soapy water, brush with an old toothbrush, rinse well, and allow to air dry.
Good old Plasticote in the jumbo economy-sized rattle can is a good primer if needed. It is inexpensive and is available in the auto paint department at WalMart or in your local auto parts store. I routinely use American Traditions Rusty Red Primer from Lowes as my finish color on resin ship underhulls. I paint the whole ship, then mask off the below the waterline & paint above