I have 10 of these guys in a 90x45x90cm, so I think 6 in there is fine. (Granted I was planning on selling a couple pairs but that's gone now) I agree with Chris here. You could have males that just aren't calling with a more dominant male in the enclosure. I have either a 6.4 or 8.2 with only three males callingThere could be a sub-dominant male in there somewhere, but either way it is usually fine to do groups and mix the sexes in any ratio for these guys.
Enjoy them, great frogs!