Stop cranking out endless reps of underhand grip curls in pursuit of bigger biceps. Do more of the stuff you're bad at and they might just start growing again. Try this:
Palms-Down Cable Curl
Why It Works
To get complete development of your biceps, you can't forget the rest of your elbow flexors and forearms. Doing more curls with a palms-down (pronated) grip offers the biggest bang for your buck when trying to hit all of these with one move.
Not only does using a palms-down grip hit the neglected (and often unheard of) brachialis, it'll help put some meat on your forearms too. And the stronger your forearms, the less likely they are to hold back your biceps growth.
Any curls using a palms-down grip will help, but not all are created equal. If you've not done much curling with a palms-down grip, then you'll be lousy at it. It's also easy to get out of good alignment with them. Your elbows may start flaring and your body will try to shift to a position it's more used to. This adds to your strengths and does nothing for your weaknesses.
Choosing to use dumbbells or a cable (and working one arm at a time) helps knock this on the head. The one-arm palms-down curl works to keep your elbows in and tight, and it hits your biceps and forearms hard in that palms-down position.
When working one arm at a time you can focus on that bicep and forearms contraction. The pump you'll get will be mind-blowing.
Try 3-4 sets of 8-12 reps at the end of your next arm workout for breakthrough biceps growth.