Your supplements are, most likely, not causing your crickets to die.
You say they come in boxes of 20 right?
Is there anything for them to climb on or hide in?
Are you providing a meal source ("gut-load", Flukker's, etc.)
Are you providing a moisture source (water crystals, Flukker's Cricket Quencher, etc.)?
These are only some of the things that might be causing them to die. The food and moisture sources being the most likely culprits.