I never was a JW, and I hope that no-one minds if I broaden this.
IME, very few 'believers' in any religion really know what they believe in. They may be able to quote a few core issues, but press them on anything even a little deeper and 'something' kicks in to deny any further discussion.
I think this applies to many believers in many religions. Many 'Christians' have no idea why or how their views differ from those of Catholics/Orthodox/original Protestants, let alone the various Christain sects along the way. Many Muslims don't have much idea of the differences between Sunni, Shi'ite or Suffi creeds, let alone the off-shoots. Many Thai Buddhists don't know how their 'brand' differs from mainstream (even if they know it does).
And that's why, IMHO, debating the basic fallacies in religion is so often a waste of time, or to put it another way 'It's a useless exercise to debate religion with the religious'.
People believe what they want to believe, what makes them feel comfortable - largely because of where they were born and the religion in which they were raised. I would guess that the vast majority who profess beliefs really have no knowledge of what they believe other than the basics. And that's why it's difficult to engage these people.
Just my view.