Help - 94 GMC Sierra - How to tighten exterior / side mirror

My drivers' side exterior mirror will not stay in position. It's the type that has a Torqx screw at the bottom of the part that attaches to the door. I can tighten it, but just slamming the door a few times and its soon flopping around again.

The part that keeps working loose is not the mounting thing that holds the mirror assembly to the door. I think whatever holds the mirror in position is stripped or worn out.

Thank you in advance for your suggestions!

Have you tried loctite?

Thank you, I have not tried Loctite. I hated to mess with it too much since I'm not sure exactly what the problem might be and didn't want to risk making it worse.