Welcome to the Forum!
I always wash the parts in dish-soapy water. Don't usually have a problem with paint adhesion, but years ago I used to.
For me, I think it was the thinner.
Are you brush painting or airbrushing? I use an airbrush, and I thin Testors paint with lacquer thinner. Used to use Floquil's Dio-Sol, but they discontinued that. I don't normally now have a problem with the paint lifting off.
Normally, enamel sticks pretty well since it reputedly "bites" into the plastic rather than simply lying on top of it.
Other folks here may have some other insights for you.