When playing on the upslope, your weight is more on your trail foot making it difficult to get up the slope into your lead foot. The tendency is to back up and hit it thin, fat, or pull. A trick to correcting this is to put the ball a little further back in your stance. The ball will tend to come out higher and shorter. The greater the slope, the higher and shorter the shot will come out.
Play the ball more towards the center, move to your lead foot, and try to hold your finish. This will encourage you to move up the slope.