From working with 1,000 + Mac Book Air units mid-2011 11", we have seen this problem many times, this is more than likely caused by a low battery or a miss configured SMC. The most common fix is to go ahead and power off the machine, then plug it in. Take note of the light, is it green, amber, or very dull almost not lighting up? Hold the LEFT SIDE Command, Option,P,R keys while simultaneously pressing the power button and letting only the power go, the light on the MagSafe connector should light up green momentarily then switch amber. Let this charge till its green then start the device. This should result in a better response.