It is most likely that the thread (crown tube) has been worn. However, saying, I have encountered on many occasion that the o-ring inside the crown has been worn and gummed up, those problems will cause frictional issues as well as screwing-down capabilities. I will firstly get a watchsmith to look at the crown - mention what i've stated here and see if he can find the problem. Either way, once you mention it, the watchsmith should be able to tell you how much to fix - instead of telling you to leave it with him and then give you a overpriced quota.
For the DIY guys-You can order new crown tubes for around $30USD.