This scenario might sound familiar to you. You can return serve very well, but you are inconsistent and tentative on your groundstrokes. Why is this?
Most players, when returning serve, stand on or near the baseline. The baseline and the service line are 18 feet apart. So, you have 18 feet of time to return a hard serve. Do you see very many players standing 3 to 5 feet away from the service line to return a hard serve? I doubt it.
Why do you think you can return your opponent's hard and deep groundstroke by standing on the baseline and taking it right off the bounce. The professionals might be able to do that. Yet, they do not do that most of the time.
Try this on a return of a deep and powerful groundstroke. As you see the ball coming start moving backwards very slowly. Just like in "dodge ball" try to get "off" the ball. Move back slowly. Swing hard or take a full stroke at the ball. Do not stop as you hit it. Aim down the middle until you become more accomplished. Then, hustle back to your ready position on the baseline.
Remember, as you move backwards several things will happen. It will force you to maintain a fast racket head speed. You have a choice to drive the ball or lob it if you are in trouble. You will not be where you can play an "out" ball. Also, this will force you to hit up on the ball or to hit topspin.
Many times, I wonder why there are just a few young Americans highly ranked in the world. Why? We grow up on hard courts. Conditions are pretty consistent and predictable. We get lazy taking the ball on the rise near the bounce. Enter into this picture all the other players who grew up on clay courts, (Europeans, South Americans), etc. Watch them! Look how they move away from the deeply driven balls. This takes care of the famous "bad bounce" on a "chewed up" clay court. They can return a very difficult shot very consistently until they receive an error or a weaker ball to attack.
Try this sometime. It will also help you return a very powerful serve.
Good luck on the courts!