It is common for weight lifters to make only marginal gains for Bicep curls. This is primarily due to improper form. To make the weight easier to lift, lifters end up using their back and shoulders, and thus not isolating the biceps for maximum reward.
The proper body form while doing the Bicep curl is to keep your back straight and shoulders back, minimizing shoulder and back involvement. As you curl, hold your elbows to your side and do not lift them up during any moment of the curl.
Applying correct form will often come at a price, as you may have to lower the lifting weight by 5 pounds or so. But it will allow you to obtain optimal bicep growth.
A method that you can use to diversify your bicep workout is curling with one arm 3 or 4 reps, while working out the other arm by holding another dumbbell midway. And then switch arms after the one arm has completed 3 or 4 reps.
As you will see, this can be extraordinarily difficult and most certainly require you to lower the weight. It is, however, extremely rewarding. And diversifying exercises is one of the most powerful tools to making dramatic gains.