short answer: too much curling, not enough pulling.
you gotta get away from isolation so much, get to some meat and potatoes work. pullups, chinups, rows.
if you are putting in 3-4 exercises: a) your intensity is too low, b) you are training for endurance (ever seen an endurance athlete?) c) you probably don't do much in the way of a negative.
my best recommendation would be to prioritize your biceps for a while....and do less work on them. for direct work i'd say do a rest/pause type regimine for a while ala DC training.
basically, do 2-3 warmups of increasing weight, on your "work" set, you are going to want to pick a weight that willl allow you ~8 reps with an explosive positive and a 3-4 SECOND negative. you go to roughly 8, which should be the last you can get with clean form. here is where it gets fun. you rack the weight, rest for a 20 count, pick it up again, 3-4 more reps, same style. rack it AGAIN. 20 more count. pick it up: 1-2 final reps. done. immediately go into a bicep stretch and hold it for 1+ minute. thats it. leave your biceps at the rack.
recap: 8 reps, rest, 3-4 reps, rest 1-2 reps, stretch. 3-4 second negative, exposive (as best you can) postive.
its going to be lighter than what you are working with now (for a little while at least). when you break out of the reps described, you add a small amount of weight to keep you in that range, you may vary slightly on the last 2 but if you are banging out 10-12 reps in the first set, its too light.
once you get ahold of this, you won't need 3-4 exercises with multiple sets. in fact, doing this type of rep scheme/technique, you should be able to train your biceps (or other muscle group) twice a week with minimal time spent. key element is to control the negative, and get into that stretch after to speed recovery and stretch out the entire muscle.
hope that helps.