How to get my SO to give me oral

So, I don't know how to go about getting this to happen. I've brought it up a few times and she said that she'll do it but it never happens. I understand it's not really her thing but I feel like once a month isn't askin to much.  Other than that our sex life is great.  I used to go down on her all the time but haven't in a few months because it wasn't being returned. Not sure what I should do here? Any suggestions?