Depending on how the pipes are run, the bleed valves may not be in the right place to remove any trapped air...trapped air, and the water has trouble flowing. The higher up, the worse it is - the pumps used generally can't overcome the head pressure unless all the air is out. The boiler may be set up to only come on when there is both a call for heat, AND the operating temperature drops below the low point. So, if there is no actual water flowing in that loop, it may or may not be cold enough for the boiler to turn on. Zone one is working, so the return water is colder, and triggers the boiler. There are numerous ways it could be setup, and this is only one of them. I still think it is probably air.