I had one long term partner who wouldn't let me and that was a big disappointment - I still don't know why, every time I asked I'm sure she never gave me the real reason... too late to ask now though.
I used to think I was god's gift to pussy eating for a couple of reasons. First was my ex wife who had never orgasmed other than on her own despite seven years of long term relationships with three guys. Another was an ex who hadn't managed to cum in company since she started taking anti-depression meds several years earlier.
Both of them needed very rough tongueing, nibbling and sucking to get them off. Both had those classic hard to find, almost non-existent "it's supposed to be round here somewhere" sort of clits.
Other women seemed kind of less into it and then I met a woman who positively disliked it, which was a shame, when turned on she had a beautiful glistening half inch long clit like a tiny glistening cock head that I really wanted to suck. I eventually realised I had to totally recalibrate my technique, avoiding direct contact, even focusing on the tops of her thighs but even then she usually said "just fuck me" after a minute or so. This was cool though as she came every five minutes or so from actual fucking. It seems her clit was just way more sensitive than the women i'd honed my skills on. On the rare occasions she did get into it I could get her squirming by breathing on it or gently pulling on her long pubes with my lips.
That said, i've since fucked a woman with an even bigger clit who came even more easily and often from fucking and she loved being eaten roughly.
I think the key, that i'd missed, is to work up to it gradually, holding off on the real 'eating' until it becomes apparent that that's what she wants. Really looking forward to getting to know another new pussy one day...