Cause #1
You may be thinking on slice shots that the racquet face must be open and coming under the ball. Your racquet is simply way too open or laid back which sends the ball up and floating.
Solution A: Instead of such an open face racquet think of the racquet nearly vertical or tilted back a maximum of 10° to 20°.

Cause #2
Every degree that you lay the racquet back you must increase the angle at which you swing down to meet the ball. As an example, let's say that you make contact with the racquet open 20° (20° is a lot). You must start higher than a shot where the racquet is open only 10°. Remember, the more that the racquet is laid back at contact, the higher you must start.

Solution A: To make sure that you get the racquet up high enough on the backswing, you've got to use the non-hitting hand and arm to help. Learn to raise the non-hitting elbow and hand on the backswing, this will ensure that your racquet is high enough. You can now increase the angle at which you swing down. As you make contact, hit through the ball and come back up on the finish, somewhat high. Remember, every degree that you open the racquet face the more you must increase this angle.

Two Key Points To Concentrate On During Your Swing:

1. Use the non-hitting hand and arm to get the racquet up above the contact point to increase your angle
2. Only slightly open (10°-20°) at contact

Fig. 50 Many times players think of the racquet face laid back or open way too much as in this photo. This will simply send the ball up and floating.

Fig. 51 Notice the non-hitting elbow and hand are up high above the ball, this will enable you to increase the angle at which you swing down to meet the ball.

Fig. 52 At contact the racquet should be nearly vertical or tilted back a maximum of 10° - 20° as in this photo.