In our kitchen, we have under cabinet lights. The plastic covers keep falling down. The covers click into place just fine. But after a few days they fall down. Is there a way to secure them? Obviously, I don't want to use super glue.
I guess that some clips or other parts that make this 'click' work are not catching firmly and/or are bending as because of temperature. I would try to adjust them if possible (if these are like metal parts) to grip more firmly.
If that doesn't work You can either remove these light covers and make a complain in a shop You ordered them (as it shouldn't be a proper way of behaviour) or use less elegant ways to make them stay in place (duct tape on other side - not visible to You). Super-glue or something isn't welcome.
You might try cutting strips of 3M adhesive Velcro, if there's a suitable place for both the hook strips and loop strips to be sufficiently concealed.
Alternatively, you might be able to put something behind the 'clicking' hook so that the hook doesn't withdraw from the cover. (I'm thinking folded paper, or rubber, or foam)
After that, depending on style of the cover, I'd try to find or make a wire spring loaded to hold the cover in place. The friction at the two ends of the wire should be plenty to hold a plastic cover against the force gravity, hook or no hook.
They loosen over time, and it's annoying. Ultimately, the only true fix is to replace the light with a new one that has its initial grip, and hasn't been removed for bulb replacements over the years. However, to avoid adding something to your light lens that may be affected by the heat, just a trace of beeswax applied around the interior of the outer trim will add some 'sticky' which is usually enough to keep the lens in place. Not too much... just a little. If using the velcro idea, or any other approach which adds material affected by heat, consider changing your bulbs to LED. (This will also keep you from getting burned if you touch the light accidentally, it keeps heat out of the cabinet, it allows you to set things directly under the lights if you need to, and it might even save a dollar's worth of electricity. The LEDs used to be much dimmer than the halogen, but they have gotten much better.