Why does my gloss drie flat?

not stirred enough? Sometimes, too much thinner will cause you to loose gloss. What was your thinning ration roughly?

I thought 50/50. I never have figured out or kept track of the exact mixture.

I'll start by saying it is possible to apply gloss paint so that the result is flat, and it is possible to get at least a semi-gloss finish with flat paint- it is in the technique.  To put gloss paint on flat, you need to use very thin coats (not much paint, regardless of thinner mix) and higher pressure.  Conversely, to avoid a flat finish you need to apply the paint thicker, and with lower pressure, to get a good gloss.

In fact, I frequently describe putting down a good gloss finish as a session of brinkmanship.  You build up the coat to a point just before it runs.  Now, how do you know it is just about to run before it actually does?  Experience.  You need to practice to learn what that appearance is.

For good gloss, I do thin just a bit more than I thin flats. I use a lower pressure (15 psi), reduce the distance from airbrush to surface, and apply it heavier so it flows on the surface.  Yes, occasionally it will run.  In that case, when thoroughly dry I sand out the run and apply another coat.

I use enamel, which is easier to get gloss finishes with.  However, thick coats of gloss enamel take forever to dry.  So I built a homemade paint drying oven. It only runs at about 105 degrees, but a day in the drying box dries as well as about three or four days in a humid cool basement.

My Dad was a auto body man for all his life. Infact he was one of the best auto spray painters around. This isnt bragging, it is fact. He never had to worry about where he would find a job. Every garage around here wanted him to work for them.

Okay, I seen my Dad do this a thousand times I think. When he had a run in the paint, he would immeditely pick up masking tape and lightly touch the sticky side of the tape very lightly in the paint run. then lightly spray over it again. I've done this on my models and it works like magic. You can't see where the run was and here is no sanding it down after it dries.

cool tip! i hope i never have to try it but knowing my cackhandedness i probably will (lots)

Don't forget about humidity, that little pest can cause some truly ugly side effects to paint. You think everything is ready to go, then along strolls HUMIDITY! Can easily be corrected-but can be a problem.

I've found that low temperature also reduces gloss, especially if the temperature is dropping.