I think the main problem is the first Robocop already told the best and most compelling story with this character-- his creation and Murphy's gradual reawakening from within-- and already had the best villains (in Boddicker and Jones) and coolest action sequences (Robo vs ED-209, Robo vs the people who killed him, Robo vs his own police department, Robo vs an entire freakin warehouse
of armed bad guys...)
The sequels could just never find a way to top all that. RoboCain might have been a cool design, but his origin was awful and it just wasn't nearly
as fun watching Robo go up against him as it was with ED-209. And about the best the third movie could do was pit Robo against an.... (ugh) robot ninja.