If I'm understanding what your problem is, I might have had this issue a while back and found out that it won't let you exchange unfinished leves until they're pretty much you're last resort. IE: If my leve list is at 6/8 I won't be able to exchange them, but once I hit 8/8 and have to get rid of some it would let me exchange the unfinished leves. You might also need to get rid of all your already cleared/exchangeable leves before it lets you exchange unfinished ones, too.