When I just tried to attack the Morkanaught (if that is right, the big beasty from ork) with my wraith guard instead of the attack symbol appearing to right click the attack, the embark symbol appeared and the squad said "the web way will take us". There were no gates or any other units anywhere around or behind the unit for it to be confused.
The wg then happily trudge toward their death without firing a shot. I madly tried again, then again and finally lower down the beats I finally found the attack symbol. This answers some questions in previous battles where I have asked for an attack then skipped over for another micro only to come back to see my units walking up casually to their death.