I've got to agree with AJ. I really think that springtails and rice flour beetles are a good supplement for froglets . . . especially those that seem rather uncoordinated or finicky.
Let's face it, even healthy froglets are rather awkward at hunting when young, and I've seen them get frustrated with flies when they would eat the living daylights out of springtails. Additionally, the rice flour beetle larvae don't move as much, and are more fatty, so those that they do actually get down contribute to that little "Sumo Wrestler" look that I strive for. Tinc froglets have a super fast metabolism, so I think that any food that helps keep them plump is fantastic.
As for the size of your flies, are you using melanogaster? How old are the froglets? I think I would definitely separate the skinny one and really throw the "grub" to it . . . and get some springtails or termites.
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Homer W. Faucett III, esq.
Purveyor of Trivialities and Fine Nonsense