Thanks for the tip, I gave that a try. I moved the connection from the valve to a different spot on OpenSprinkler and it worked.
I think the problem might be with the valve – if it sits for awhile without being activated, it needs to be triggered a couple of times to start. Then it works right for the rest of the day when I test it. If I let it sit overnight then it has the problem again. I think there may be some debris stuck in it.
It turns out there are lots of YouTube videos on how to repair and flush your sprinkler valves, I’ll give that a try next.