Could be seasonal but at 400 gm that is quite small for sexual maturity usually that is not until at least 600gm usually closer to 800-1000 for males. You never know they are individuals I think it is just a big meal I offer the adults about 5% once a week and they (breeding season omited) usually always take it mine isn't a huge group just 11 regulars and 2 rescues at the moment (they are rehab animals and come and go when I am sure they are healthy) It is not a exact science lots of ideas few right and wrong ones.
I'll try a smaller one next week and if he still won't eat the week after that I'll go to every 2 weeks till march or so and try again.
BTW, sounds like you're doing some good work....
1.0 Lesser (Lemmy)
0.1 Enchi (Baby)
0.1 Firefly
0.1 Fire