a quote from charles poliquin:
“The Scott bench is used to isolate the medial (short head) of the brachialis muscle. A lot of bodybuilding jock kinesiologists, however, will say that a 90-degree inclination on the padded surface will work the lower biceps. Too bad there’s no such thing as a “lower” biceps. If you feel soreness after a Scott bench workout, it’s because you’ve worked the short head of the biceps brachii and the brachialis. Since the distal insertions of both of these muscles are in the crook of the elbow, people invented the term “lower” biceps.”
you should do exercises that target the short head of the biceps. keep in mind that you cannot specifically isolate one part of the biceps and that a lot of your problem has to do with genetics. we were all born with varying insertion points. try reading poliquins “variations on a theme” article. hope that helps.