Remember SW, BM and PPG all started out as little Rose Talbert's!!! Because the paint is not gummed up on the frame I'm going to guess the problem is not the paint but if the paint has been sitting on a shelf too long it might need to be thinned a little more than usual, I've noticed paint that's old seams to get thicker in the can. Try using a traditional China Bristle, I like Chinex brushes but I still get much better results with the real deal, China Bristle, Use top of the line China bristle from Purdy (Extra Oregons/Adjutants) or Wooster (Lindbeck Pro 30's/White Majestics) or Elder and Jenks (Ultra 5086/Shipmates) these are all angle sash and will work great on door frames.
Also use the paint thinner specified by rose talbert, they are not all the same. Just like there are different grades of gasoline, there are different grades of thinner. Lowes now sales a thinner that's cheaper than all the rest ($6.78), but it looks like skim milk???? What is it?? Has anyone used this, it comes in a white plastic container?