The phenomenon when you can’t plunder resources 

even though having successfully attacked an enemy dominion, occurs when 

either the enemy dominion has no resources or resources little enough to be protected.

However, you can check the plunderable resources on the scouting report if you scout ahead

so make sure to scout before attacking.

※ Scouting may be limited or may fail depending on the

level difference between your Lighthouse and the enemy’s Castle.