Thorns bugged or no?

[City Builder]

This has been bugging me since starting to play. All of my thorn creatures require a fallen creature in the cell next to them to be able to spit up a new thorn. However, the opponents thorns can just spit up new thorns as they please, regardless if they have any dead creatures in the cell next to them or not.

So I figure it's something I need to learn how to do so I click on them to get more details & the tool tip tells me that they too are supposed to only be able to use that if there is a dead hero in the cell next to them.

What am I not understanding here that allows the opponents thorns to use that feature to spit up a new thorn but my thorns can't unless there is something dead in the cell next to them?

Thanks,