Oh jeez, I can't believe I didn't respond to this originally.
I'll do some testing to see what I find out.
You've said that you haven't found a pattern, but that some Event Categories work and others just don't. Are there any specific categories that regularly fail?
Sure, on our site (benethugs.dkpsystem.com) the event category called "Zul'Aman" won't Quick Add. (In fact, none of our ZA categories work with Quick Add.)
However, the one called "Karazhan Thug Team" works just fine.
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Quaiche - Addon Author, Druid, and Guild Leader for Benevolent Thuggery on Dragonblight