So, has she not had an orgasm from penetration either, or your fingers, or her own?
Assuming she is somewhat orgasmic, you might try the following:
Tell her before you go down on her that this time you are going to focus on your own enjoyment of getting you face full of her beautiful, fragrant, exciting goddess parts! Then take your time doing just that and slowly build a mental contour map of her sex; learn the scenic routes, the short-cuts, the sweet destinations. Tell her that every touch, lick and glide is you loving her, to relax and not hunt the orgasm, just sink into the feelings and enjoy you enjoying her.
Make happy noises. Wriggle. Have fun down there and she might catch the same ride.
It also helps if you can stop thinking and just lose yourself in your senses and emotions. This can take practice.