toughpup94 saidyour biceps arent big enough to have flatlined..
Exactly. 14" at 6' tall, the OP may need to re-evaluate his lifting methods. He's definitely not small, but has reached that point where he needs to work harder to grow. What's worked until now now longer works...gotta step it up from beginner to intermediate/advanced mode now.
As jocksportsgear points out: does the OP just swing the weight? Or does he raise it, concentrating on his bicep actually working and contracting? You wanna concentrate on the bicep working, not on the weight being raised.
Also, are you eating enough?
Are you eating immediately post workout (something that can digest quickly such as whey protein with sugar for quick energy to repair the muscle)?
It's awesome that you're incorporating different set and rep ranges. Do you ever do cheat reps (after a full set of proper reps)? EVer do drop sets? Do you control during the negative portion of the rep? Do you feel the pump? Do you ever do reps for the burn?
Also try different angles. Arms away from the body (concentrated curls, preacher curls, scott curls, high-pulley cable curls)...arms behind the torso (seated incline bench DB curls, drag curls, behind-the-back cable curls)...different grips (wide grip barbell, narrow grip barbell, straight barbell, EZ barbell, reverse grip barbell/DB, hammer curls)...and....
most importantly...do you hammer your triceps as well? That's 2/3 of your arm right there. For a lot of newer guys, it's not intuitive to hammer the tris as well. But once you've gotten them to grow, you'll realize how nice, meaty, and balanced your upper arm looks now, and it fills that sleeve nicely.
Cheers...and don't hesitate to ask follow-up questions.