Give 'em a little more water. Mist them with a spray bottle every night, stop if the frass gets damp. (Which it shouldn't.) They can survive dryer conditions than most roaches, but prefer moister conditions.
Do you remove all the frass when you clean? If you do, leave some. Nymphs love burrowing into frass.
How much protein is in the dog biscuit? Consider upping the amount of protein you feed them- I'm talking cat food. I did an experiment where I fed nymphs different diets, the cat food ones not only grew at least 3 times faster than the oatmeal ones, they also grew faster than the control nymphs that had access to cat food, oatmeal, frass, and sugary cereal at all times. I don't know for sure that more protein would increase reproduction, but it would definitely increase nymph growth. It seems that the more protein you put in, the more protein you get out.