I felt like i had this problem too until I changed my rune setup. Use ap/lvl marks and glyphs, maybe seals too if you feel that your positioning skills are good enough.
I build my boots first, then unholy grail -> rylais -> deathcap. Dont try to harass before about lvl 5, just sit and farm, and after about 10 minutes youll start dealing good damage, after 30 minutes the damage really gets going.
Even if you dont go for the item build, definitely try the rune setup, my ahri play improved instantly