Quick question - is this a finecast model?
Follow-up, did you scrub/clean the model with soapy-water first?

sometimes the finecast stuff still has a bit of mold-release residue left of it and that can cause paint to not stick to the surface. It doesn't matter what primer you use at that point, it just won't stick. a quick scrub with an old toothbrush and soapy water will take it right off. Hope this helps.