How much exercise does he get and how did you train him to like his crate? It sounds a bit like he's not accustomed to being in it for long and there's more to it than just put him in and leave him to figure it out on his own. Gotta start with very short sessions and gradually move on to longer when he gets used to it. Some dogs like some toys to keep themselves occupied as well, like a kong or treat ball.
Personally I started crate-training the very same day I took my dog home at 8 weeks. He got all his meals in his crate, played fun games in and around it and he's slept in his crate every single night since then. The first few months every time he fell asleep elsewhere during the day I carried him into his crate, just to reinforce the idea that his crate is where he sleeps, and now he's perfectly content being there and just sleep. I do think my dog was quite easy to crate train, I expected much more troubles (and sleepless nights) than I had. I can imagine crate training an adult dog is harder and will take more time.