I've got no idea but it seems that you are unsatisfied and really communication is what's important.
If they are unwilling to open up about this or grow in this area it just might not be the right relationship for you.
So, brave person who has come to a great place to respectfully help the situation, I have to ask, what is stopping you from opening up this communication at this time?
Are you scared because you don't want to bring it up, (which I totally understand)? But let’s be real, you won’t get your answers here. You need to take that brave step and open up that communication with her.
First thing’s first, do not do it right after sex or before or during. Bring it up on a peaceful day. Kind of when you’ve just been hanging out for a while and having a nice time and there’s no special event happening.
So what I would say, or what I would want to hear, would be:
Hey, can I bring something personal up with you?
/yeah/
I want to talk about something that has come up for me, but I just want to start off by letting you know that [positive feelings about your relationship, like I love you so much or something, or if you haven’t said that say something else like I really like spending time together] and I want you to know that I didn't want to bring it up because I did not want to bring up any problems for you.
/Let her respond to this /
You can start slowly talking about how you have felt like in bed, lately, and have noticed that?
I don’t know her personality but she might take offence here (not your fault) or might not. Might have noticed it or might not have.
If she has noticed it- ‘Do you think there might be a reason for it?’ and the conversation can go gently from there. If you notice her getting angry or offended at any point, I would probably say ‘hey maybe we should leave this conversation for later’ and just give things some time to cool off.