I got an underweight 1 year old BP on August 7th. When I got her she was only about 130g. I started her right away on weaned rats (around 35g, which were about her width), she had been on ~30g adult mice up until then, but I imagine she was fed infrequently due to her size. When I got her she was in shed, but the guy told me it had been a long time since she last shed. She shed and pooped (at the same time, which is common) a day or 2 after I got her, and has continued to take 35-40g rats every 5 days. She shed a second time the week before last (maybe 6 weeks after previous shed), but she was then at around 350g, so that was a roughly 220g weight gain. As for the pooping, she has passed some marble sized urates (maybe 5 or so), but only pooped during the sheds (so about 6 weeks apart).

How much does yours weigh? I would say you definitely have to move up to at least adult mice. Have you considered switching to rats?