Well, this is making less and less sense all the time. It doesn't seem to be a collision problem since I moved the door to float in mid-air and it still highlights yellow.

Odder still, when I deleted all doors except the first one it of course worked perfectly. Then I MOVED/DRAGGED the same door and trigger to another part of the map and it highlighted yellow (yes even tried the floating in mid-air test again).

So as little sense as I can make out of it....it appears to be some kind of geography problem...which really, makes no sense at all.