MacBook Air charge plug light not working
According to a tech who works at a local Apple repair center it happens mostly on older plugs. But on newer and current plugs it's also not unseen.

Usually unplugging plugging back the magsafe fixes the problem. If not the problem is usually fixed once the computer is powered off and the plug is reconnected.

Personally I have seen this on most of my macs. It never bothered me much. But on one of them this used to happen all the time until the battery was replaced.

Usually the problem is the green LED being turned on instead of the amber LED during charge. But there are occurrences of none of the LEDs showing during charge.