you could lower the weight. alot of people "dig"their feet into the floor to force weight and cheat, addign stress to less reinforced ancilliary muscles and connectors.i alternate betwenn seated and standing.make it a new staple of your bi trainign and give it time, especially if you are natural or not very lean, which i'm guessing you are or are not respectively, if you don't have that pumped look 24/7 in a relaxed state.works wonders for thickness of the bicep and strengthening the brachiallis will improve your gains in other excercises as well