My husband is passive in that he doesn’t usually initiate and usually only does the things I ask him to do. Sometimes I can tell he’s horny, but if I don’t say anything, he won’t either, and then he assumes I must not have been in the mood. One time I waited to see what he’d do, then the next day asked him, “I thought you were getting horny yesterday… You didn’t want to have sex?” And he said, “Yeah, but you didn’t say anything, so I figured you didn’t want to.” 
He’s still working on initiating… He does have one thing he started doing recently when he wants a blow job 
I’ve found that during foreplay and while we’re having sex, if I ask him to hold me a certain way or play with my nipples or kiss me, he will, and then after a few sessions of asking for something, he’ll start to do them without me asking him to. During blow jobs, once he’s feeling good, I make him tell me what to do next since I have his cock captive (to help him communicate what he likes because otherwise he just won’t tell me). That all being said, most of the time when we’re having sex, I lie back and let him do all the work and heavy lifting 
Your girlfriend might not know that you want her doing anything different from what she’s doing. You could ask her to do specific things. Make sure she knows when you’re feeling good, and she might keep doing those things. Ask her how she likes things as you’re doing them to her. Communication is important (even if it’s just body language or your this feels good noises) 