My girlfriend doesn't like oral sex! why?

She says doesn't find any pleasure on it. The thing is that I just love to do it and I'm getting a bit frustrated that all the time she says no to my attempts...
I always felt pretty confident doing it, never had any complains so now I'm a bit confuse.

I guess I need some tips, please?
Should I try different things or?
My girlfriend doesn't like oral sex! why?
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