I have similar issues where even primer won't stick well or more commonly,, when you shoot it, it lays on the car 'splotchy'. Not sticking in some areas, but sticks on others.

I have had this happen on many Moebius models, but on Revell too and most recently an AMT Mustang.

Cleaning a model I have never had the problem.

I think some batches of models just seem to have more chemicals on them or it doesn't get washed fully off at the factory.

If I didn't wash a model first, I test the primer or paint on a small part, or the underside of the hood (which would be easy to strip if needed.

But more and more I just wash the whole thing.

Anyways that is my experience.