1) No one should have sold/given you leos that young. That is the sign of an illegitimate breeder, if you ask me.
2) Check your husbandry. I bet your temperatures are off, they may not have the right hides, they may be on the wrong substrate, they may be housed together, they may be being fed the wrong feeders.
3) Not all vets can detect parasitical or bacterial infections with one fecal. Some are inexperienced, some are experienced but don't know what to look for, and some won't find anything unless your leos are tested several times over the course of 1-2 weeks.
4) They should be gaining weight and growing rather quickly, so something is very wrong - go through a check-list and see what you might need to change in terms of housing, feeding, and fecal checks.
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Too many Leos
1.0 feline "Spot"
0.1 canine "Tika"