The most common reason is water intrusion.The water comes in and sits in the washer that holds the spring on the pot metal shaft corroding it until it breaks under the pressure of the spring.
The fix is to replace the whole assembly,If they are Velvac mirrors the part # for them is on the bottom of them,this will help in get the right parts.
This company will help you out,they have a lot of Velvac Mirrors and parts,look around on their site you might find something that would work for you.
Pacific RV Mirrors.Right Mirror:
If you are handy you can replace the mirror yourself,I have replaced mine it wasn't very hard to do.
This
Link posted by CloudDriver will walk you thru the repair.