BSFL taking forever to turn into flies.

[CutThroat Kid]

I recently got some Black Soldier Fly larva (maggot looking things), in hopes of feeding them as flies to my mantids and jumping spiders. But I've had the larva for over a month and only gotten two or so flies from it, and that was over two weeks ago! Everything I've read online says they should pupate in about two or three weeks, so I'm lost here. All the larvae are alive and healthy, eating coffee grounds, organic material, and food scraps in a tall deli cup with about two inches of substrate/meal. They are kept room temp or warmer. What's going wrong here?

 

[jbooth]

I heard they love Slipknot... maybe blast that for them I seriously haven't a clue

 

[jbooth]

The first thing I'm seeing, they want to bury in dirt to pupate, this guy has it rigged so they climb out and he transfers them...

. I've never messed with them before but I'd like to know in case I ever do.

 

[Isopods others]

Put your flies in your airing cubboard with a damp paper towel, I do that for my blue bottle castors works a treat