Entry Sensor Malfunction?

I have an entry sensor on my backdoor. When the system is set to home or away mode, opening the backdoor does not trigger the alarm countdown - it doesn't even recognize the senor as being tripped. It's only when I close the door that the alarm countdown starts. Basically, if someone tried breaking into our house via the backdoor, the alarm wouldn't go off so long as they didn't close the door. This has been happening sporadically over the past month or so. Anyone else experiencing this issue?