engine

Three possibilities, either you are not fully tightening gas gap all the way. This is easy fix to make sure you hear the click when fully tightened. The gas cap or filler neck sealing area is defective and not sealing properly. Then a new cap may fix it. The third ids there is a leak in the system somewhere. The gas cap light is just to remind you to properly install the cap. The system is basically testing for a leak and can't always tell the difference between a loose cap and leak elsewhere in the system. Actually there is a way it can infer a cap leak by the timing if it happens right after a fill up. However if the system leak happened to show up right after a fill up then the system sees it as a cap problem even when not. You should have the system properly tested by a qualified tech to find and repair any system defects as fuel vapor leaks are an environmental and potential safety hazard.