The most common reason is that the battery is failing or the power is switched off on the socket. If this happens when you're setting up your system, it may be that it hasn't picked up your sensor.
For the door and motion sensors check the batteries (and that the power is on) and then press the circle on top of the gateway (hub). The sensor should flash red as the system picks it up, then it will stop. Your sensor should now appear as active.
With the smart plug, make sure the small button on the left side of the plug is on (it will show a green light).