airplay not looking for devices

I have a problem with my macbook pro.

Some time ago I turned off airplay becuase it kept looking for new external devices all the time. Now i'm not able to turn it on again. I selected the option in display preferences but even though the box is checked the icon won't show up in the menu bar. I also forced it starting from system/library/core services/menu extra/ displaypreferences.menu and this makes it showing up the icon in the menu bar, although as soon as i click on it, it vanishes. IS there a bug? what can i do? Now my mac can't recognise any plugedin hdmi devices or minidisplayport.