I wouldn't worry about it too much as it's mainly but not entirely a genetic thing. The potential that IS there can be brought out by a few things:
-Incline DB curls using a lower incline of 20-30 degrees.
-Use an adjustable preacher if you can find one (better but no longer popular) and adjust the pad to be close to or horizontal. The pad will be flat or close to flat-you want your elbows in a raised position, up higher than usual. Then make sure to lower the weight all the way down to the bottom then back up-that will exaggerate the effect on the lower bis. If this type of preacher's not available, there are some preacher machines that use a flat or almost flat pad, use those.
-Partial standard preacher ROMs, doing only the bottom few inches for an entire set or at the end of a set. Be careful though, this is stressful on the connective tissues and can cause injury if the movement's not done in a controlled way or if the weight's too heavy.
-Chins, just do the first few inches of ROM.