I have a Rachio 3 with flow meter. When I installed and calibrated last year, everything worked great.
This year, part way through the watering season, I am constantly getting low flow alarms on every zone. And when I try to calibrate, I don’t get a flow reading. But the watering works fine; just getting a lot of alarms, and no data on flow.
I changed batteries, deleted flow meter accessory and re-initiated. Accessories indicates flow meter is connected. As I have a fair amount of pipe, I have the settling time at 3 minutes, same as it was last year when it worked fine.
Still a problem. And it is the case with all zones. Pictures below
Assuming the meter is not actually broken, there are a couple things that could be happening. A vortex meter requires about 10psi minimum back pressure to ensure no air in the pipe and that that pressure wave can get from the vortex shedder to the sensor. In about 99% of sprinkler zones, you won’t have a problem attaining this. But, having long lines, lots of heads, higher flow rates, and a 3/4" line are things that can make it more likely to be a problem if the meter is mounted in a downward or horizontal position.
I just saw a situation last month similar to what you describe where 8 out of 10 zones stopped working and the owner turned up the pressure from ~45psi to about 50psi and all 8 zones started working.
So, if you can try turning up the pressure that would be one place to start.