Backhand slice lands short?

Question

I can hit an effective backhand slice but it always lands right on the service line. I can't make it go any deeper.

When I try to get under the ball more, it floats and lands out

Answer 1

your follow through is too short.

Answer 2

For the backhand slice: the slice spin comes from the racquet moving downward at contact with the ball (the face is about vertical at contact).  For the depth, it is the speed of the approaching ball,  racquet head speed, and forward movement into the ball.

The reason your ball floats is because when you attempt to get under the ball your racquet face is opening more instead of being vertical (1 degree of change can alter the ball up to 6 feet in depth).

Without seeing your stroke, either you are chopping to much downward and need to go more forward to flatten the swing or your swing is too slow and you need to produce more head speed.