Check your connections back to the engine module. If they are good my guess is the sensor.
Dirt can cause the same problem. Not sure how to clean, I wouldn't use compressed air, you could try blowing air or running fuel reverse through it and see if anything comes out. Hold it over a cup with a paper town to strain the fuel.
The first -8 I built the next owner reported the same thing. FF was erratic for about a month, then went to zero. The sensor (red qube) was engine mounted. I replaced it, adelled to the engine mount and has worked since. I noticed on the old unit the vibration sleeve on the wires was broke free, and figured the direct engine mounting was the cause. I know others have reported the internal flow sensor gets excessive wear from vibration. I am assuming yours is engine mounted. If so I recommend moving it.