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You could have accidentally disabled it.Try to enable it and see if that occurs again.
Does the same occur in safe mode?
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008
Try to run the phone in safe mode for one day,see how it is.
It could be just a glitch.Will you try what I suggested?
Ok,report here after.