Check the DC Voltage on the RED wire going to ECO WHEN you turn Propane ON.
(be set up READY to measure then have someone turn it on....circuit board only sends voltage to gas valve for 8-10 seconds while sparking. Then shuts down voltage if no main flame proves)
Sounding more like circuit board issue.....could be bad grounding.
Green wire from circuit board to frame at exhaust shield
Green wire from circuit board to wire nut connection at back of WH Tank to DC Neg
Spark electrode assembly mounting screw...ground source for spaker