I, too, had this problem and found that the only solution was to reboot OpenSprinkler. I also run HomeSeer, and developed an event to recognize when OS was offline to warn me.
I tried any number of approaches, including an access point that was just 10 feet away. I finally gave up and switched to OpenSprinkler Pi. The Pi has a much more robust TCP/IP stack. Haven’t had the problem since, and the access point is now 20′ and two walls away. (My original OS installation had a wired ethernet connection to a TP-Link powerline connection. That worked just fine, although I really dislike using powerline for communications.)
Incidentally, I’m impressed by the reliability of the wifi connection on the OSBee. I have two of those, both out-of-doors, one of them using solar power and the other powered by the 24VAC sprinkler power (bucked down). Even in the worst of winter here, -20 degrees F in the early morning, both of these continue to communicate.
Rog