Why do my black soldier flies die?

I bought soldier fly larvae in mid-April (about 500), I put half of them in the standard compost bin without caring, the other half in a tray to check their reproduction. The larvae fed and, once they turned black, they collected themselves, so I moved them inside a cage / mosquito net and covered by a bucket waiting for them to become adults. I have never seen a single adult! some pupae are visibly dead, others perhaps still alive, but it surprises me not to have ever seen one become an adult, nor have I found dead adults! since they arrived, the outside temperatures have almost always been above 20 degrees, in the few days of bad weather they have dropped by a few degrees. The only adult I saw by chance was hovering around the uncontrolled compost bin that I hardly ever control, but the larvae whose reproduction I wanted to control did not seem to develop. In YouTube videos everything seems so simple, what can it depend on?

The aim would be to self-produce worm casting for use in pot cultivation (I am a beginner and I am trying to learn) and use black soldier fly to create food for worms. I know it's not a necessary step, but using the feces of hermetia illucens seems to me much more efficient, I didn't know it was so difficult.

Are they covered by the bucket completely? It sounds like they might not have enough air to breathe?

If you are able to post a picture, we could better see what you are describing.

Hi! Thanks a lot for the answer. I have attached the image to better understand. it is a makeshift arrangement, nothing definitive, I needed it just to experiment and gain confidence against this magnificent animal. just today I saw the first adults! I think that keeping them outdoors the temperatures were not optimal and the development was a bit slow. I think, however, for my climate I cannot enjoy the BSF during the winter (I would also like to compost indoors with their help but unfortunately I can't really!). Although here - southern Italy - I read that they are in hardiness zone 10 and outdoor temperatures rarely drop below 4 degrees, in the winter months I imagine that the activity of the larvae becomes too slow and I would have problems with reproduction.

I'm looking for some BSF-like animal that can go well even for the winter to pre-treat any type of organic waste in a way and then give the resulting feces to earthworms.