I'm queueing one geared out 80 dk with my 4 poorly geared shaman. The 4 man team can all queue as a group no problem. There are only three eligible dungeons, all having ranges going up to 80.

There is no other clue though as to why they are denied.

I'll try some more combos tomorrow, 1 tank, 1 shaman, etc...

hmm...oh, it was the level 80 queueing us. That was most likely the problem.