Try some lube or cleaner down beside the stem, road dust and grit can collect in there. Even soapy water can help.Ok, makes sense. That's how it's been with the mirrors on other bikes I've owned too. I just wanted to make sure before I put these in a vise and try to free that nut up. Both are so stuck that it's going to take some significant force to free them up. I didn't want to break them in the course of trying to budge something that wasn't meant to budge.