I have my crickets in a 40 gallon glass aquarium with plenty of areas to hide. I have corn flakes for food and for water a moist pape towel. When the crickets die I always take them out to get rid of the ammonium created. Also, I have vermiculite as substrate to get rid of excess waste. I have the crickets in my garage which provides plenty of heat(so I don’t need a heat lamp). I have 3 crickets die every day that are medium sized and haven’t reached full grown length!! I even check if it’s shed exoskeleton and it’s not. I’m confused please help.
Crickets can be tough to keep alive. It sounds like they have water but not too much, and enough food. They have a place to hide which is good. Maybe try a little bit of squash or something like that and see how they do. Changing their food source might help. Keeping them warm definitely helps. That said, I ran into a lot of trouble keeping them alive for very long myself and wound up switching to roaches after a while because of it.