Strange problem with unloading a world...

Ok, I wrote Hunger Games plugin. It works but today I get very strange problem.
My plugin before every game unload the HG world, copy it and load it again.

The problem is that bukkit can't unload the world. I really don't know why.
I tried to unload it manually with My Worlds but it said:
Failed to unload the world (main world or online players?)

Ok, I'm sure. There are no players there... There are no loaded chunks or entities.
I checked with Essentials: /lag command return:
World "HG": 0 chunks, 0 entities.

But I can't still unload it... I'm really confused...

Any idea why I can't unload the world?

 

there may be conflicting plugins that cancel the world unload event

 

Ok, now I put this in my plugin

Code:

@EventHandler(priority = EventPriority.HIGHEST)
    public void onWorldUnload(WorldUnloadEvent event) {
if (event.getWorld().getName().equalsIgnoreCase(plugin.cfg.getString("hgWorld")))
if (event.isCancelled())
event.setCancelled(false);
    }

Will see if it will bug again.

 

If a plugin has put that world in a list or something it could prevent the world from getting unloaded.