I have 3 bps that were very slow growers--all ate every time with no problems but barely grew at all in a year's time. Then one passed a tapeworm. All 3 turned out to have internal parasites (all 3 had come from the same source and I had the rest of my collection tested and they were all clean, thank goodness). Once treated and retested as clean, they all started gaining weight immediately. I'd recommend fecals for your slow growers if you haven't had them done already, just to rule that out as a possible cause.