I' sure you have, but I'll ask anyway. Have you made sure the corresponding weapon is in your armoury chest, and hevae you checked every armoury slot, head, feet, etc. I have had this happen to me as well and sometimes, while it may look like I have enough space, I actually don't, try removing any armour you don't need (not in a set) and then change classes, that may solve the issue. I know sometimes it was the ring slot that screwed me up.