How many do you have in a container, how large is the container, what is the water temperature and how often do you feed and clean?
If they're overcrowded, container too small, not enough waterchanges/spot cleaning and/or temperature is too warm or fluctuates too much within 24hours, this can stress the larvae which can lead to fungus developing pretty fast and subsequent death in some cases.
Larvae are by far more susceptible to water quality/temperature fluctuations than adult axolotls who may take a couple of days-weeks longer to develop similar fungal symptoms.