There are so many factors that can influence whether or not your girlfriend initiates sex. It’s important to understand what it is that’s stopping her. Does she lack confidence? Does she fear rejection? Or is it that she thinks for some reason it is taboo for a woman to make the first move? Is it because you’re always the one who initiates it, and therefore she doesn’t need to?
It could also be linked to mood on any given day. Or maybe she doesn’t have as high a sex drive as you. Because there are so many influences, the best thing is simply to ask her.
"Simply complaining that she never initiates it will only discourage her more."
If it’s confidence or a fear of rejection, what would help is for you to make her feel really comfortable. Simply complaining that she never initiates it will only discourage her more. We all respond much better to praise than criticism. Encourage and empower her, giving her lots of positive reinforcement by praising her when she does something that you like.
Different sexual desires can be trickier to manage. If she has a low sexual drive it might indicate something else in the relationship that needs addressing - it might be that you need to consult a psychosexual therapist to understand what that is.
Most of it comes down to good communication. Talk to her. Encourage her. Be kind and patient.