dear Vicky, you may have to adjust the balancing valves in the supply loop of your system. these will be in the basement of the house. If you look at the pipe comming from the supply side of the boiler and follow it to the firstradiator connection there should be what looks like a garden faucet(for draining the system). Next to it should be a small slotted screw head. This is for the balanciing valve for each radiator in each zone. That means there is one at each radiator connection. It is a simple butterfly valve, when the slot is straight up and down it is closed(90%) and when it is horizontal it is all the way open.The range of adjustment is only a 1/4 turn.You can use these to devert water for each radiator. remember that the radiator that is first in line will always get hotter that the rest. Also with any hot water system there in natural rising of water(convection) in the system.
If you find the balancing valves for the zone that is too hot you should be able to cut them back. As far as your thermostat, check to make sure that the boiler responds appropriatly when you turn it up or down.