These mirrors were on my bike when I bought it. Last week I tried to take the left one off to mount a GPS bracket to it. It was turning hard but was turning so I thought it was loosening then the bolt just snapped! Major bummer! Upon inspection the bolt was coated with red Loctite. I have the same replacement mirrors on order and now seeing your post am wondering if I'll have the same problem. Apparently Loctite was needed to get the ones I have now tight. Maybe try some blue Loctite which is supposed to be able to allow you to loosen again. That's what I'll do if I can't get my new mirrors tight. Let us know what you end up doing that works.
BTW I think the teardrop mirrors really look good on this bike. This was the perfect chance for me to go with something else but I'm simply replacing with the same as I like them that much.