I believe its all genetics. My best friend and I both played football in school since 7th grade. He has a much much better peak than myself, while my biceps are much thicker and rounded, his look to have a golfball stuck on top. Its been like that before we lifted and stayed like that till today and we are both 21. I do preacher curls, hammers, seated curls, standing curls, standing straight bar curls all using supersets etc.. The only thing that I have gotten is a really good bulge at my lower bicep that really makes them pop out, but still no peak.