That's exactly the problem, I've already collected everything. I'm out of money to buy more troops. There's nothing left to do in this part of the map, and these elephants are just standing there, not moving, not being lured out. Maybe it's a bug? I've been wandering around there for a month, if not more.
It would probably be more appropriate to post this question in the "Technical Support" section. A possible solution could be to replay the mission from the beginning.
What version of the game are you playing? Is it a licensed copy?