This has happened a few times now, where i am attacked, I have 5 defenders and only 2 of them are brought into battle. The rest of the units have been in town for quite some time, and are never triggered. The units which are able to be brought in are Mirror Elemental and Young Mirror Elemental. 2 Spiders and a Wraith Knight are in town, unable to defend.
If it helps to solve the issue, my town swallowed up a death mana node next to where I settled.
