Why did my right wing mirror stop tilting automatically?

Usually when I put the car into reverse, the right wing mirror automatically tilts downward enabling me to see my right back wheel and immediate area. However, this stopped happening after a day of particularly heavy rain. After a few days the seemed to resolve itself and the mirror functioned as intended until two weeks ago when the same thing happened after another rainy day.

I am still able to fully adjust both mirrors within their usual range using the joystick. I've also checked the gap around the mirror with a flashlight and can't spot any dirt or rust that might hamper it's function.