Questions about the gameplay of Heroes 6.

I've encountered a problem. The computer-controlled opponents in the campaign don't seem to want to play. I've seen many posts from people complaining about how the computer is aggressive and constantly attacks... but in my case, it shows very little activity. When I approach them, they sometimes haven't even gathered resources, let alone try to take territory from me.
The difficulty level is set to maximum. Can anyone help me understand what's going on?