This happens pretty much every day. I log in, my spell crit is at 29% then after an hour or so it's down to 27% and slowly drops. Just now after 2 AA trial runs I went from 29% to 21% on my character sheet. All gear is repaired and my staff is at 100% charge. Some days when I log in I'm only at 27% or 25%. I have all the light armor passives and am wearing a willow path set, warlock set, and worm cult set. Only the willow path affects crits. Thief mundus stone never shows up on my character sheet either since I've zoned a few times since I grabbed it.