There are several ways to circumvent this annoying bug.
The first is the most time consuming - You have to delete the unit and put it back in again and then just skim through the triggers and see which ones are red, and reselect the object, and then it should work.
The second way is to go to the units section and select the player that the unit belongs to and go back and reselect the unit and it should work, you can always hear when the unit belongs to the selected player beacuse they will speak when selected.
By the way do not delete the trigger, just use the same one when you come back to the trigger section, this will save you about 30 seconds work.