This is actually a common problem as most people initiate the curling action by curling the wrists in, especially when nearing fatigue. This will improve leverage by engaging the forearm flexors, but reduce the work of the biceps muscle. What you need to do is cock your wrists backward while curling to increase the work of the biceps and effectively eliminate the forearm flexors by putting them in a weak position. You may need to reduce your curling poundages initially, however, in time, you should be able to return to your normal weights. You will also have bigger biceps to show for it!