Lob the service line not the net man
To get your lobs deeper, lob over the service line not the net player.
If your lobs are consistently landing short, focus on making the high point(apex) of the lob happen over the service line.
Most people focus on lobbing over the net man. This makes the high point of the lob happen over the middle of the service box, or roughly, where the net man is standing. With a couple of steps, the net man can move back and hit the overhead from the service line. Not a desirable out-come for you.
By lobbing with the service line as the apex, you make the net man shuffle back to almost the base line to hit the overhead, or better yet, chase the lob and hit a defensive shot back.
So remember to lob against aggressive net players, use the service line as the focus.