User blog comment:B214/Volume 17 spoilers./@comment-90.176.222.50-20140612151706/@comment-4829046-20140612155505

Also cause Goku was mostly one who received power, not always give it to others. Issei offers more support unless of course it is something he needs to do like with his major foes. Goku likes taking everyone out by himself, the only reason he doesn't do so when he has the chance is since he feels others should also enjoy good battles. And cause Saiyans can level up by themselves, all they need to do is get beaten or angry and instant power-up. Sure maybe not super power-up but power-up nonetheless. Issei is more gradual and more reliant on others rather than always pick it up on the fly. We get dragged out moments where he feels bad for his lack of power.