It is possible that you cooked it too long. Overcooked chicken can be tough and rubbery, hard to chew. If you use chicken thighs, this should not be as much of a problem. But chicken breasts are picky and can dry out quickly, causing them to become chewy and tough. Make sure you cook them just to 163 degrees F. They will continue to cook as they rest, so they will be perfect at 165 degrees F when you eat them. Remember, dark meat is more tender than white, so I use thighs.