Some apps remain on top (and I think they keep stealing focus), so OD doesn't appear when I move the pointer to the edge. Coriously enough, it works with Chrome but not with Thunderbird.
Same problem with Directory Opus.
I can't say I've noticed that with Dopus [and I don't use TBird].
Stealing focus [or not relinquishing it] is likely the cause though....I'll have to have a closer look...