MBA doesn't automatically detect display connection

my mba (13" 10.7.2) has developed problem doesn't automatically detect when external display has been connected or disconnected. example, i'll open mba up, connect display through thunderbolt connector, , won't activate external monitor until tell ''detect displays," , when disconnect it, seems have no idea needs return mba display being primary. i'm not sure if hardware or software issue (as works fine 90% of time after give "detect displays" kick).

 

i getting same thing lot lovely, working past few months. started missing display recently. btw, other ports work fine on display, not screen until hit detect button in preferences, or reboot.

 

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