The first thing you should check is whether the station is set to activate the master station. So click on the ‘Settings’ icon next to the station to bring up the station properties dialog, and see if the ‘Activate Master’ checkbox is on or not.
Also, please note that ‘Use Weather Adjustment’ is different from ‘rain sensor’. ‘Use Weather Adjustment’ means the watering time of the program will be multiplied by the watering percentage. You said ‘when rain is detected’ — I assume you have a rain sensor connected to OpenSprinkler. That’s independent of ‘Use Weather Adjustment’ — a station can be set to ‘Ignore Rain’ even if the program where it’s in doesn’t use weather adjustment.
Other than these, I can’t think of any reason why the master is not activated. The way master station is handled is different from other stations — it’s not affected by the rain sensor directly, instead, the algorithm turns on the master station if any station that associates master station is on. Rain sensor only affects non-master stations directly.