Did you start with adult rats? Maybe too old and that is why production is low?
If it were mine I would stock one or two females per tub and a male for a week or two. Move the male when they are gonna pop and don't move a male into a tub till the babies are weaned at 4 weeks.
Personally I DO NOT run a rack system so I can't judge tub size. I run rats in 18X18X24 (lXhXw) inch cages with a wheel and 1.4 usually. Every month or two I move males around or move in a new young stud. I probably maintain too many males sometimes as you can get away with very few but I let them grow and breed and then use them as large feeders for my boas. Most of my snakes are as yours....small rat eaters.
When I set up a new cage I put in prime medium rats, not larges who may be past prime.
-----
Sonya

I'm not mean. You're just a sissy.
Happy Bunny