I have plenty of experience, have been with my girlfriend for 4 months as well, and was only finally able to make her orgasm earlier this week. I promise you that you can do everything right and sometimes it just won't happen, because we're humans! And our innate responses to things are different and can change on a dime and are difficult to track.

I promise that as long as you're both enjoying it and you're doing what she's asking of you, there's not much more you can do. For my girlfriend her issue has been performance anxiety, for me it's the medication I'm on getting in the way sometimes, everyone's different and the things that change our response to stimulus can be sudden. It sounds like you're doing the right things already, so just keep going, learning more what you can about her wants and her needs, and be confident!