How do I stop my under cabinet light plastic covers from falling down
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.