There are a few reasons why this is happening. One is a reaction of the new paint to the old paint. Another can be contaminates. (Silicon spray is really really bad stuff with paint) Another reason is adhesion. There are adhesion promoters that you spray on first then apply your paint/primer. Another is the wrong type of paint. Also I recommend sanding with 320 grit. I used to use 400, but found that the 320 promoted better adhesion.

My recommendation is to remove all existing paint (use 180 grit) down to bare metal/plastic. Clean surface with a wax and grease remover. On the plastic use the adhesion promotor (Bulldog is one) and prime everything. After it dries, sand with 320 grit and paint.
I have used the Krylon plasic spray with some mixed results, and it sounds like you want a really good finish so I would recommend not using it.