Let me start off by saying THANK YOU to the Rachio support team for helping me today debug a lingering issue with my system. I have posted my issue and the resolution so others can learn from my situation.
Upon moving into an older home last Fall, I installed a Gen 1 unit, replacing a very old controller on an existing 12 zone irrigation system. I set up my zones and programmed my watering schedule - everything worked great.
When I recommissioned the system this Spring, I started to run into an intermittent issue - I wasn't able to manually turn zones on. Sometimes the system worked fine and other times it wouldn't. Rachio would register my request to water a zone, but the solenoids wouldn't operate. Both the Rachio unit and all the electrical components of my irrigation system were operating correctly, but for some reason the Rachio wasn't signaling to the solenoids properly.
The issue turned out to be a ground wiring problem. The unit I replaced had the rain sensor (not clearly marked as such) wired in-line on the zone solenoids' ground wire - so, when the rain sensor was dry the ground circuit was complete; however, when the rain sensor was wet the ground circuit was broken. Since I simply moved the ground wire from the old controller to the Rachio, I unknowingly kept this rain sensor ground circuit intact. Thus, when rain sensor was dry the Rachio worked fine; but when it was wet, the whole system was not operational, and I couldn't manually operate any zones. Since I didn't typically water (either scheduled or manual) when the rain sensor was wet, I didn't experience the problem very often and it appeared to be an intermittent issue.
The fix was simple - we properly connected the zone solenoids' ground wire directly to the Rachio Ground terminal and connected the rain sensor to the Rachio rain sensor terminals. Problem solved.