DOES NOBODY HERE BREED MEALWORMS??

I still have a few hundred large mealworms, still not pupating. I order 3000 medium mealworms, received 3000 small, they send me a new shipment so now I have 6000 small/medium/small in one container and a few hundred large in another. Tonight I grab some mealies out of the small container for my geckos and what do I find? Pupating mealies in the SMALL container! I find about a dozen and seperate them, still NO PUPAE in my large mealies! This is what happens, I buy them and bring them home, they pupae like crazy for a couple weeks than stop and no more pupating. I'm not finding any large number dead, they are all thriving and growing, just not meta-morphing. ANY IDEAS??
Thank you!