The mirror is self tightening as I recall.
Here is how to detach and reattach the mirror.
CLICK See posts2 and 4.
Take it off and see what the problem is maybe the attachment is broken or bent

 

There is a link below that shows photos and how to remove the mirror. I did as instructed but it would not budge. I was afraid that I was going to crack my windshield. I sprayed WD-40 into the hole in the back using a w-d straw, I let it soak for a few minutes and then it twisted right off. I did use the allen wrench to help turn the mirror. I wanted to remove my old mirror to install a new dash cam mirror. Good luck to you.

 

I had this problem also. The mirror wobbled side-to-side while still attached. Others were correct about how to remove, just turn the base 90 degrees. Once removed, you will see an odd shaped, metal fitting in the base. This is the piece that creates the pressure fit with the mount that is permanently attached to the windshield. This metal fitting is connected with a torque screw. I figured the mirror wobbled because the metal fitting has become sprung, and I was correct.

I took the fitting and heated it on a gas stove burner (holding with pliers). Then I crimped in the sides ever so slightly so that it would fit snugly to the mount on the glass. It was a little bit difficult to re-attached - probably because I overdid it with the crimping. Eventually got it re-attached and it fits nice and snug now.

 

Great tip, thanks!

 

Thanks I'll give that a try. Seems like such a weird attachment for a mirror! Once removed I guess I'll have to crimp on the spring attachment to tighten it up. Appreciate the help!