Why One Wish Willow works for some and not others
A theory about the One Wish Willow that I haven't seen anywhere else yet (or maybe it's out there and I've just missed it completely).
We know for sure the OWW works for some and not others (due to the web/forum search Bear does). Maybe some wishes are just too outlandish and can't be fulfilled... OR maybe there's a cost. The common thread for all of the wishes we see come true is death; in every instance there's a life-cost just before it's made. It feels like Sandy the cat was a catalyst to the first wish being fulfilled.
Bears wish = Cat dead
Ian's wish = Sarah dead
Fake Nicki's wish = Ian dead
Whether the death is caused by you or just surrounds you (knowingly or unknowingly) the death being so close to your own spirit could be the reason the One Wish Willow works for some and not others.