1. Adults take communication seriously. I guess your gf isn't adult enough for you to talk about sex. Which means you will never solve the problem. You can try to talk to her, but if she won't talk about it, there's nothing you can do.
2. Some girls just don't like giving head.
3. You are a bit large, and some girls have a small mouth. Some also panic if they can't breathe, which is why they don't give head. (An old gf told me that's why she had so much trouble giving head.)
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
(Please note, we give the best advice we can based on the information given in a post. For better advice, please include the age of all romantic partners.)