If you don't see crickets actively on the food source, maybe they can't reach it. Wrap piece of paper towel around outside bottom of plate or food tray so they can climb up to food easily. It's also not a bad idea to supplement your crickets' grain diet with some fruit occasionally. A quarter of an orange or apple every few days is just the trick. Water should be also be available to them at all times.
Crickets can drown easily in standing water.
Many factors come into play here. Second is the age of the crickets when you get them. Temperature, nutrition, and cleanliness of the storage environment are also big factors. This does NOT mean that all crickets will live that long. Crickets often go through some degree of physical stress during the production, loading and shipping processes, which does shorten their life expectancy.