If the ground is dry and rock hard it is hard to dig. I always used a garden fork to loosen the ground around the vegetables and work them back and forth (again gently) before trying to pull.
Also try to harvest before a big rain (especially after it's been dry) because it could cause potatoes to have hollow centers. Onions need to dry before and after harvesting. My carrots grew in the flower beds edge and I just pulled them as needed. They did very well staying in the ground. Let all your potatoes, onions and garlic cure in a warm dry place before storing. If you're going to cook, freeze, dehydrate or can this probably isn't essential but things like sweet potatoes taste better after curing awhile.