I read somewhere online many many years ago (a reasonably reputable source, I have to say) that the sensation of a female about to ejaculate is often like being desperate to go to the toilet - that feeling fit to burst - and that many women find this sensation offputting which is why they rarely ejaculate. Many times they mistake the ejaculation sensation for actually needing the toilet and so 'clamp down' so as not to indulge in unexpected water sports thereby killing the ejaculation.

Different women have different sensitivities. Some take a lot of effort even to get a normal orgasm (yes, even when doing it themselves) while others explode almost as soon as touched. Others find the sensation of clitoral stimulation actually painful - even a light touch can hurt - but can easily orgasm with a vaginal or even labial stimulus. So, female ejaculation is equally rare for some and common for others depending on an individual's sensitivities and willingness to 'let go' during sex. Many women will never have one (just as many have never had an orgasm) while some will have them all the time. Just a symptom of natural variation.