Forearms are a lot like calves. For one thing, if yu have high insertions of the extensor (top of the forearm) and/or the flexor (underside of forearm) muscles, it's harder to build mass - similar to having high calves. Also, forearms tend to respond to higher reps, much like calves, probably because they have a lot of endurance-oriented fibers due to the extensive use they get in everyday activities.
So, what's the solution? Super-sets or tri-sets, which not only build muscle but also increase vascularity. Keep in mind that a vascular forearm appears much more massive than a smooth one, even if the vascular forearm is smaller, so try to get the veins to surface.
You should also work the brachialis, which can help beef up the top of the forearm as well as heighten the peak on your biceps. Here are a couple of result-producing routines.
Aftershock Forearm Routine
Hammer Curl, 2 x 8-10 ->
Rope Cable Curl, 2 x 8-10
Reverse Wrist Curl, 2 x 8-10 ->
Forearm Rockers, 2 x max
Wrist Curl, 2 x 8-10 ->
Behind the Back Wrist Curl, 2 x 8-10
Triple Aftershock Forearm Routine
Hammer Curl, 1 x 8-10 ->
Rope Cable Curl, 1 x 8-10 ->
Rope Cable Curl (lighter weight), 1 x 8-10
Reverse Barbell Wrist Curl, 1 x 8-10 ->
Reverse Dumbbell Wrist Curl, 1 x 8-10 ->
Forearm Rockers, 1 x max
Wrist Curl, 1 x8-10 ->
Behind the Back Wrist Curl, 1 x 8-10 ->
Forearm Rockers, 1 x max.
Those are pretty extensive forearm routines, so you may want to cut back on other parts of your workout. For example, you could reduce your direct biceps work somewhat, as the brachialis exercises will also indirectly target the biceps. If you can't handle cutting back, you may want to try doing only one set of each of the exercises listed in the first routine - but really push them hard, working through the pain barrier. You have to punish the forearms to force new growth.
To do Forearm Rockers, stand, holding a dumbbell in each hand with your arms hanging down and the dumbbells next to the outsides of your thighs, pointing forward. Contract your flexors, curling the dumbbell toward your body as hard as possible. Then reverse the movement, taking them up away from your body as high as possible to contract your extensors. Do that till you can't hold the dumbbells any longer, but rack them quickly before you drop them on your feet. Pick a weight that has you screaming after about 45 seconds . . . and stay close to the rack.