Thank you for your question.
You are not alone in finding some less than tender chicken breasts. There are currently 2 issues facing the poultry industry - "woody breast" and "white stripe."
Woody breast is found in about 10% of chicken breasts. It can be hard to detect with the naked eye, but the meat may have a paler than normal color. When touched, the raw meat may feel tougher/harder or "knotted." After cooking, this chicken may taste "woody" or "gummy" and be hard to chew/cut. Much of the meat with this condition is identified during processing and is used for ground meat.
White stripe looks exactly like it sounds - white stripes of fat running with the grain of the muscle in/on the breast meat. However, this does not work like marbling in does in red meat. The white stripes actually make the meat drier, less flavorful and less able to absorb marinades/flavors.
These issues are not a health risk to humans - nor do they make the chicken less nutritious - just less tasty.
Both of these issues seem to be, in part due to the fast growth of commercial poultry. To try to avoid it, look for poultry from slower-growing or heritage breeds, if possible.