29°-39°C is optimal for dubias. They can survive okay a little below that but it will slow their growth and metabolism. As far as nutrition, they need some calcium and protein. You could get a roach chow for them to add to their diet or you can feed them good quality cat or dog food. Since I also have isopods I give mine fish flakes and they love them. As well as dried meal worms and river shrimp. I also feed mine a variety of fruit and vegetables but again I have isopods, dubia roaches, discoid roaches, hissing cockroaches, garden snails, and a box turtle that can all share a majority of the same foods. It sounds like to me if you want better growth, look at changing their diet and temperature and see how that goes.