have you kept track of pee and poops. If they poop on regular basis they will loose wait to where it looks like they havent gained any.

If your doubting everything take in a fresh stool for a fecal float done to the vet to check for parasites.

at 160 grams you can be feeding adult mice every 3-5 days. at 100 grams you can feed sm adults every 3 days