I had the same problem before. I also had a group of three young Cynops ensicauda popei but one died for the same reason. The other two ate fine, and put on weight but no matter what I tried the smallest one did not. He ended up dying from it, but luckily the other two have done great. They are a few years older now, adults, and they both have a little bit of a belly on them
I think they are a fantastic species, possibly the best one I have had.