I agree with everyone that communication is important! Is it something she might be interested in if you could just figure it out? Or does she just not want it and feels like you’re nagging her? If she doesn’t want it, maybe try to find out why without pressuring her. Knowing her reason could help you put it to rest. When you do bring it up, be careful not to sound accusing even if you’re being sincere and not intending to, like “Why do you never let me go down on you?” “Why won’t you let me?” “Do you think I’m not good at it?” - these all could sound like you’re blaming her 
It could help if for several sessions, you don’t get your mouth in there at all, and maybe only focus your mouth on her inner thighs while you’re using a toy or your fingers on her. Or give her some kisses on her labia and leave it at that while moving on to something else (of course if she’s consenting to it). Hope you guys will be able to have an honest conversation with each other 
If she does want it, this is a great thread to help with your oral techniques 
