The real-time flow count uses a 30-second window. If the flow is so slow that it doesn’t even click in 30-seconds of time, it will end up being calculated as 0. You can modify the window size by going to defines.h, and change the flow count window size:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/defines.h#L63
to something larger, like 60 seconds, or whatever that guarantees there is at least one click within the window period.