Hi Mark
Try this test: Stand with your feet flat on the floor and lower yourself down into a squat, trying to keep your heels in contact with the ground. If you have to lift your heels up to keep your balance before you've got all the way into the squat position, then that shows that the problems you are having are due to a lack of flexibility, and not just with the dog!
Search for calf stretches on the Internet as well as quad stretches. Good luck!
George