Agree with what's already been said. Healthy nuc-sized colonies don't raise drones, and so I'm not surprised to hear that an observation hive hasn't.
LJ
Whoops - just checking: by "observation hive" do you mean the type often placed inside a building, typically with 2-3 frames ? (that's what I've assumed above)
Or could it be a full-sized hive fitted with windows in the brood box sides ? I know some Warre hives are built like this, but don't know if Langs and similar are.