I can mark patches as WIP but not as not WIP. What's worse a bunch of patches seem to have ended up as WIP without anyone (I know of) explicitly marking them. This is pretty bad as zuul seems to refuse merging WIP patches even if they are +2-d (also they are hidden from some dashboards and notifications).
@Addshore says he has a "start review" button to un-WIP so probably it has to do with gerrit permissions.