Halterbreaking a Stubborn Calf

I have a seven month old calf I bought about three weeks ago under the impression that he was already halter broken. The first two weeks I had him he was great, he would walk fine and came right up when you would go into his pen. Now he isn't wanting to walk, I can still put the halter on him without any problems and he's fine with walking a few feet outside his pen to be tied up. I have had no problems with washing, blowing, or brushing him but for some reason he no longer wants to walk. I've tried some of the usual methods like leaving the halter on him in the pen but they haven't seemed to work, any advice?