Screen sharing stops working

On my Mac Mini running OS X Lion Server software, the screen sharing will work for a while from one mac on my network to the server, but after a week or two it won't show up. To get it to start working, I have go to the machine and turn off remote management and then turn it back on. This is rather difficult to do when it is headless. I logged out of MobileMe, but it still does it.

This seems to have started a few months ago after a Remote Management update. I haven't noticed this when accessing my Mac Mini's running standard Lion and Snow Leopard.

Is this a bug? Anything else to try?

Thanks.