It can be any number of things, but without seeing you play, we can only guess. But it sounds like you may have a technical problem. Such as poor footwork, bad positioning, inadequate follow through, you may be making contact with an open or closed face, therefore hitting long or into the net. It can be many things. It would be a good idea to have a USTA certified coach take a look at your game and see what they say. You have to have the proper mechanics in place before you start trying to hit big. Early on your focus should be on consistency and proper technique.