Possible reasons:
The trigger is not activated, or gets deactivated by another trigger.The wrong player number is selected: E.g. the unit belongs to player 3, but the trigger affects units for player 2. This should only be a problem if the trigger tries to task units in a certain area, and the unit you want to task isn't selected. If the unit is selected, it should still work.The wrong unit type is specified. E.g. the trigger tries to task all men-at-arms, when in fact it should task archers. Like above, this can only cause trouble if the trigger tries to task units in a certain area, and the unit you want to task isn't selected. If the unit is selected, it should still work.No location has been specified, or the trigger just didn't "swallow" the information when you specified it. This is an editor bug which happens sometimes. Make sure the location is selected by using the "Go to" button.The unit's path is blocked. Try to loop the task trigger and deactivate it when the unit reaches its destination.Ingo