What can be done to make my biceps feel harder?

Work them: Do different types of curls and extensions. Eat right. And keep changing up your workout routine. Your muscles get 'used to' the same work out, so it is best to keep changing the exercises that you do. Use heavier weights with less reps. From personal experience, triceps are very tough muscles to bulk up. And they are even tougher to maintain. So keep at it. Good luck.

Hard biceps. : You must do two things to get hard biceps. First, you must work the biceps muscles for shape and tone, which means high reps per set (15 to 20 reps to muscle exhaustion). Second, you must do cardiovascular exercise to thin out the layers of fat and overlying the biceps muscles (and your muscles in general).