1. Scans can be temperamental. Try ticking the 'double hrv tolerance' in System; but, if green is on and red flashing, it sounds ok.
2. If the blink isn't in sync with the blood flow to the area being sensed, you may need to relocate the sensor slightly to obtain a more reliable location as well as the Double HRV Tolerance setting in the System tab (which is only visible if "Advanced" is clicked in the Presets tab) referenced by Paul above.
For more details, please check the link: