It could be how heavy it's being applied too. I have an Optima gun that has an "atomizing control" where you can turn it down so with the same air volume and paint volume have very little atomization.
One time I painted a vintage Wayne fuel pump with the gun using a Urethane SS and I didn't want to move it and painted it in the shop turning the atomization way down so as to not get red overspray all over everything. It sprayed out just fine, a lot of texture but shiny and just fine for this gas pump. That sucker took days to cure, it was days and still in dust as I remember!
The trapped solvents were preventing it from curing. Just a thought