What temp do you keep yours at? My buddy that breeds them thought they were slow growers at first too, but unlike rats, they like the heat. He moved them to his fish room that I believe he heats to 80 degrees and now he almost can't keep up with production and they are growing ridiculously fast! If you keep yours in the air conditioned house at 68-70 degrees, that may be why they are growing slow. I would try raising their temps a little bit if possible and see if it helps.