Hm.. I can keep up to 500 1/4" crix in a 10-gallon tank..
First, I would ditch the sand. It's totally unnecessary and gross, really. It may be that the sand is doing something bad, but I doubt it. Do you keep any water sponges or water crystals in there for them? I would guess that an apple would supply enough
hydration, but maybe not..
Are you getting large crickets? It could be possible that they are at the end of their life-span.
In my tank, I keep a 'dish' of water crystals, a smaller dish with beardie pellets for food, and a small paper plate with the same greens I feed Blaze. They actually eat more greens than she does. I used to put in potatoes, but since Blazey eats greens now, I just use those.. If I have a potato or two that's gone too far for me to eat, I still throw pieces of them in.
It could also be that the crickets that you are getting are simply unhealthy, but I hope not..