I think it could be because Micheal can't find Dean.

I was thinking the same thing, but then I remembered that Lucifer visited Sam in his dreams and asked him where he was hiding. Presumably, Lucifer and Michael are at approximately the same level of ability, so if Lucifer can contact Sam in his dreams, Michael should be able to contact Dean. So, there's got to be something else we don't know preventing Michael from contacting Dean.