Vallejo makes several polyurethane gloss finishes. One takes at least 24 hours to fully cure, and reapplying it too soon could cause your problem. Frankly, I don't like their gloss products, though their satin and flat are fine. "Thinned a little" is rather imprecise. It pays to measure, though it doesn't have to be exact, so you know what your starting point was. For a properly thinned coating, 25 is on the high side. I usually spray at around 15 psi. Depending on the size of the airbrush aperture and dilution, 3 to 4 inches may be a little long, though at 25 psi it may not be. The pressure, dilution, and distance factors all influence each other. It's a balancing act.
Try to look at the surface at an angle, that may make it easier to judge how thick and wet the clear coating is.