no drag 'n drop from itunes to finder

I see a lot of quenstions about drag and drop in itunes, or from finder to itunes, but when I want to copy something out of itunes to a thumb drive, it doesn't work.

at first I can drag n drop and it start copying, but at one moment, it just won't copy. When I hover over the finder screen with my load of songs, they won't stick. for a brief moment it worked again after a restart of my macbook, but this was one time only. Then it wasn't possible again.

Anybody with the same problem or maybe a solution?