i have more or less the same problem . i did a field battle , the army arrived , and came back without doing any damage to my enemy . my complete army is comming back with no damage , but the army of my enemy has also no damage . since i checked the spy report constantly , i know that he didn't move his army . how is this possible ??
your enemy army is set
to retreat.
Command Center --> Army Deployment --> CLICK RETREAT "When attacked your army will retreat and not fight. If the attack is against the Fortress, you army WILL fight, despite the order to retreat."
your army at field will not fight if field battle or pillage had been ordered by your enemy, unless it's fortress siege they will be forced to fight