Paging @jomaxro to this topic. However there is already three other topics on this bug, so this bug might be very common:
http://discourse.stonehearth.net/t/archers-wont-shoot-if-they-had-a-defend-order-prior-to-an-attack-order/22156
http://discourse.stonehearth.net/t/archers-dont-attack-and-stutterstep/22150
http://discourse.stonehearth.net/t/archers-stand-next-to-target/22123