Yeah, I'd say two things. Your body has adapted to that program, and is no longer getting much from that workout. That "21s" workout is supposed to be a shocker plan, as in do it like once a month or something to that affect. Two, how much are you doing? Yeah, you say you can't lift another pound after, but truely, I bet that's not the case.
Try this Bicep workout next time.
1) Start with either: Standing Alternating Dumbbell Curls -or- Standing Curled (EZ Curl) Bar ~ 4 to 6 sets (2-3 of those being light warm up, say 40lbs, then 50lbs, then 60 lbs Curled Bar, then hit the 70, 80, 90 or whatever heavy stuff)
2) Seated Incline (at least 45 degrees) Dumbbell Curls, getting the most out of the STRETCH portion of the lift, at the bottom, and hit the peak, Supinate if possible at top ~ 3-4 Sets (advice: do not come all the way up to your arm pit with the DB, leaving your elbow in one spot, the DB should point at an angle towards the ceiling, not straight up, fighting gravity's pull)
3) Standing Hammer Curls w/ Dumbbells -or- Cable Rope Curls (Hammer Position) ~ 3-4 Sets
4) Finally, Standing between two Cables, high mount, Double Bicep Curls, as in doing the Double Bicep Pose ~ 3-4 Sets And this should be Failure for sure... just to finish them off. (*If you don't have access to this setup, do Seated Concentration Dumbbell Curls, with elbow inside of knee)
Perform all these sets with enough weight to be able to do NO MORE than 12 Reps, but at least 6 Reps. Meaning, if you can make it to that 12th rep, add weight on the next set, but if you can't do at least 6, take a little off. SO, IN OTHER WORDS, DO AS MUCH WEIGHT AS YOU CAN, with strict form, of course!!!!!!!!!!
Let me know how that goes for ya, if you try it!!! Oh yeah, and if you have enough energy, do some wrist curls, and reverse wrist curls, just to finish off your forearms while you're at it, in case the Hammers weren't enough.