It happened now at least 3 times on different content. When you try to matchmake in a party of 3, then you won’t find a party. It happend now on guardians, cube and boss rush.
One time it was 2 bards and 1 artilleris for guardian. Yesterday it was 1 berserker and 2 sorcs for cube and boss rush. The third time I can’t remember.
Hello, just to conclude Centeotl’s investigation.
When queueing with multiple of the same class, matchmaking appears to not work properly.
This is an issue that has been passed on to the team. Thanks!
Thank you for chiming in and helping clarify things, OminousOnion!
I hope you have had a great weekend, RWBY! 
If it’s okay for me to add on, the same issue appears when queueing into cube with 1 bard, 1 paladin, and 1 artillerist. We waited 5 minutes before cancelling and finding a 4th manually.
My guess is the bard and paladin are both marked as support in the queue. Bard and Paladin were both running appropriate DPS/mob clearing builds.