Well, go back to the first principles: You have to get their weight slightly off the foot to pull off the sweep, or go for timing number two.
How slightly off is situational; you may have to do a lot of work or not try it altogether. For instance, I mention the knee lock before I sweep: This is key for bringing down much bigger people than myself. Try it, see what you think.
Also, keep in mind that this sweep can be done at other places in combat, not just right after he punches. You can do it as they are trying to match your footwork, after they have fallen, all kinds of things.
And finally, sometimes an inner foot sweep is just the wrong technique. That's up to you to decide. What I'm giving on this video are the mechanics of foot sweeps in a wide window of application to a broad range of practitioners.
Next week: Those damn ninjas!