[Zen]

First thing: Make sure the unit isn't stealthed.

Second thing: There is a bug that if you do your turn, then reenter the game and move other units there is a slight possibility that all movements will be reset and if you don't move everything exactly like you did last time it won't get the order.

But before you jump to conclusions, make sure you didn't move the units in stealth and forgot to change them to 'move'.