The mama may not be strong enough. Maybe it's fed on a veggie-based diet with a lower protein content to save costs. More fish, meat or chicken could provide them with some more fat & protein to stack up their bodies for reproduction. Some people have seen the flies go dead after pupation. It may be because their bodies are relatively too weak for reproduction.
The black soldier flies love mating in early morning light. The light warms their body temperature well enough to be in the right mood for mating. Make sure there's good lighting around so they can mate & lay eggs. Naturally, the flies will gravitate towards bright light corners. During rainy season, although the humidity is good, it may lack some sunlight. In the winter, when they go into dormancy, they may not lay eggs. Some people use breeding lights to breed indoors in these cases.
For newly pupated soldier flies, they may not lay eggs immediately. For the first few days in their short lifespan (5-8 days), they need to find a partner first for breeding. After they mate tail to tail, the females will lay their eggs. Black soldier flies are also known to be notoriously hard to mate. But it's not impossible, so be patient & don't give up hope yet.
Also, make sure the density of the flies population in your love cage is not too much. Some people stack too much flies in one place. This has proven to be counter-productive for them. As the flies buzz around & bump into each other, they get confused. So spacing out about 10-20 kilos of fresh larvae in a 6 square meter cage would be okay.
Have you checked that the attractant is good enough? Mama soldier flies love the smell of rotting or fermented matter. This means yummy food for her babies when they first hatch. Some good attractants are fermented food, bran, soya bean, rotting fish, chicken, meat, veggies, etc. One farmer uses the dead bodies of the parent flies from previous generations as attractant. Ugh, it smells so bad but according to him it works.
Also, design some good egg space or 'nests' like a stack of wood or cardboard. The concerns of the mama flies are the predators eating her eggs or the sun would dry out the moisture in them. So a good egg space protects the eggs from ants & other stuff and gives good shade to prevent the eggs from drying out. Watch out for rats as well if you're setting the love cage outdoors.
See if there's any other stuff around. The flies may randomly lay their eggs into the safety gaps there & not your desired places. This happens a lot to a lot of growers. It's because of the smell that gets diffused around in the environment. When she lays eggs there once, she'll find that same spot to lay again & again. Because it's safe for her.
- Good food so the mama & papa have good health for reproduction
- Warm morning light so the flies can mate easily
- Some newly pupated flies may be finding a love mate
- Space out the population so it's not too dense in the love cage
- Make attractant with strong, attractive smell for the flies
- Design safe egg nests that's near a food source
- Look out for ants, rats or any other places that the flies may lay eggs