The main cause of the flow sensor to malfunction is when the installer uses Telflon tape on the cold water connection or if there is debris in the feedline. If the tape is applied too close to the end of the fitting, small pieces can enter the sensor and prevent the turbine from spinning.
 
All Siogreen units are shipped with a small screen that fits in the cold water inlet connection. Sometimes that screen gets stuck in the blue cap when you remove it. Be sure to check for the screen and install it. This will prevent most of the debris issues. After you install the unit, be sure to flush the unit completely and recheck this screen for debris.

The typical indicator of a flow sensor issue is the Run Time Indicator. If the timer is not advancing when the hot water faucet is turned on, there is probably an issue with the flow sensor.

Download the full diagram below.