When I trained Noah, I made sure that "step up" had a strong history of reinforcement. Noah almost always steps up for me, even if he's doing something really fun or eating something yummy. From day one of step up training, I made sure step up = fun. Whenever he steps up for me, he knows he's going to train, get a treat, play with a plastic bag, sing and play with water, etc.
Also, I find that Noah prefers stepping up onto my forearm. He would only step up onto a perch or my forearm, and it was months before he'd step up onto my finger. While hands and fingers can be "good", they can also be "bad", and most birds realize this, so they can be hesitant to go near your hands (yet they have no trouble trying to stick their head in your mouth -_-). See if he steps up more reliably when you offer him your forearm.