Well, sadly, some people don't run on your schedule. I still have 4 accounts not piad in full. Only one of them worries me, the rest will pay... when they get called by me next week.
I wouldn't be worried until they get back from Germany and keep blowing you off. If it hits mid Jan, just send them a letter to pay you.. or it goes to collection. If they still don't pay.. sell the debt to a local collection agency and move on.