I assume you're talking about African reds and blacks? In both cases there are more than one species being imported that are practically indistiguishable from one another without the kind of examination they aren't about to cooperate with. So the first step, matching the species could be difficult in itself. There are also very few males in the trade. The local collectors look for the mature females and they are pretty easy to tell apart from the MM's. The MM's pretty much don't even make it to the boat.
Apart from that, I think they are also pretty finicky about the conditions they need to produce a sac. I haven't tried to breed them myself but I've read a few threads about it and it seems quite a challenge. Not impossible, but not easy.
Maybe someone who's actually tried to will chime in? I believe Cacoseraph is working on a project with a local CA species of trap.
another problem is baby trapdoors are hideous to take care of. the babies are 2-3mm DLS and will make hidden burrows if you put them in normal substrate. they are smaller by far than normal fruit flies.
other mygs that don't really make trapdoors are easier to care for, but the actual trapdoors freakin suck