Humidity has less affect on an enamel paint than an acrylic. If fact, it mostly affects the initial drying time, and generally will have little, if any, affect on curing time. There is a surface effect caused by the hygroscopic nature of some polymers, which will slow curing, but this does not last more than a day unless something else is wrong—like attempting to paint over it. This may be the whole cause of your problem.
Your gloss is not curing properly. The first question that comes to mind is: How long did you wait before overcoating with the gloss? The second is: Were the paints under the gloss also MM?
Not waiting long enough to overcoat, especially with a clear, will cause solvent still escaping from the underlying paint to alter the behavior of the clear, and possibly cause problems with the underlying paint, too.
If the underlying paint was not MM, then there is also a possibility of incompatibility.