One tip I can offer is to soak the plastic for a while to get rid of the releasing agent on the plastic. That could help, especially if you're using acrylics. These oils on the plastic may reject the paint. As a practice I usually soak all the sprue trees from a kit overnight using dish detergent. I know other modellers use other products like brake fluid, but I've never needed more than just detergent. Also make sure your paint hasn't settled on you with the pigment seperating at the bottom of the jar. Give the jar a good shake and stir with a toothpick prior to painting.