Do you thin your paint??
Acrylics got bad adhesion on straight plastic, especially when the plastic has not been washed, on some types of plastic it will not stick at all.
Lay down a coat of primer first and than use a soft brush for applying the paint.
Thinning it with a thinner that has an extender mixed in will also help extend the drying time and help prevent brush strokes in the finish.
I think the Tamiya acrylic thinner already has the extended mixed in.
HTH.