Glad the crate is working well for the two of you Linnie.
I've yet to find a pup that couldn't be crate trained no matter what breed or of what mixed heritage however, there are some older dogs that no matter what......just cannot be crated. As was already mentioned in part, some of these dogs are shelter dogs, their history is unknown and crating just isn't a good idea when they attempt to injure themselves.
Yes, there are things that can be tried to get them used to the crate but not every dog can be retrained if they've been faced with severe traumatic experiences.