My girlfriend has never let me go down on her. Why?

Firstly, I’m 18 and she’s 20. Our relationship has been great, not any red flags. But one thing that has me wondering is our intimacy. She’s not scared to initiate sex and I’ve gone down on her before. But she’s never let me eat her out. I’m obviously okay with this because I’m happy to pleasure her, but it does sting a little bit when I ask her and she doesn’t want to return the favour. What could be the reason for this?<br><br>EDIT: not *i’ve* gone down on her before, I meant *she’s* gone down on me before and vice versa.