I also suspect the paint is drying midair.
I don't use much spray can anymore, but Tamiya rattle can primer is about the most forgiving rattle can model paint out there. Since you mentioned you have trouble painting, it could be techique.
In a nutshell, press the nozzle and start your sweep off the model. Use a smooth steady motion and never stop, continuing off the model before you release the nozzle. Repeat on the next pass, overlapping at whatever works with the previous one. That's really all there is too it. If it is rough, you are too far away. If the coverage is too light, you are sweeping too fast. If it pools or turns 'glossy', you are going to slow or you paused.
Eagle already hit on the other variables, wind, temp and humidity. I've sprayed tamiya outside in some pretty unforgiving conditions and can't say I've ever had a problem.
If this is too basic, I mean no harm. Since you stated painting is troublesome, I just figured why not make sure the basics are covered.