Dropper Bottle Won't Close?

Hi all, just received a bottle of AV yesterday and for the first time I asked for the dropper. Doses out great, the only problem is that when I screw the dropper cap back on to close the bottle, it unscrews itself.

I know this stuff is pretty magical, but what's going on?
Anyone know how to fix this?

EDIT:
It seems as though the threads are slick and the rubber that holds the dropper applies just enough pressure to keep it from staying closed. I took the dropper apart and wiped everything dry. problem persists.

I'm afraid my AV is going to evaporate or spill. Has anyone else experienced this?