P90X Day 64: Chest, Shoulders & Triceps + Ab Ripper X

I thoroughly enjoyed Chest, Shoulders & Triceps this morning. My form continues to improve, and both my reps and weights are increasing. I'm positively killing the Tony-style airborne plyo push-ups, and I'm now able to squeeze out a respectable number of one-armed push-ups, too. I'm even getting the hang of the various triceps moves.

But thank God I didn't have to do any pull-ups or chin-ups today. I'm having a hell of a time with them. Although I'm now able to gut out almost twice the number of unassisted pull-ups as I could on Day 1, I've been stuck on a plateau for a while.  My rep count hasn't seen much upward movement in weeks, which frustrates and bewilders me. Why is it that I see significant progress in my ability to perform virtually all of the P90X moves except for pull-ups and chin-ups?

I don't have Back & Biceps until the day after tomorrow, and I'm already dreading my rendezvous with the pull-up bar in my garage.