Check Bluetooth on your iPhone
Make sure your Bluetooth capability is turned on in order to connect to the sensors.
To do this, go to: Settings > Bluetooth > select (green is showing).
Check your PostureTracker sensors are active
The PostureTracker sensors can only be detected when they are blinking blue once per second. If your sensors are removed from the PostureTracker charger and are not blinking blue, they might have gone to sleep mode to save battery. Simply move the sensors a bit to wake them up again. The blue blinking should start immediately after moving the sensors.