Gas and Oil Home Heating Furnaces - No heat in one zone

I've a 3-zone gas heating system (baseboard). Since last week, there has been no heating on the ground floor whereas the first floor is heating well (the basement heating was not on but it's not working either). There is one circulator pump and three zone valves. I tried switching the thermostats to see if they're the problem; the one from the ground floor worked on the first floor. That excluded the thermostats.
Ground Floor: When the thermostat is turned on, the valve opens up (the lever on the zone valve motor falls loose) but the furnace doesn't fire up. I changed the switch on the zone valve (bought a new honeywell zone valve (8043E1012) and replaced the electric portion of the old valve only). The furnace fired up but the temperature on the ground floor has not improved. We checked the supply and return pipes for the zone... one loop is warm, however the other loop is cold on the return side.
Any advice would be highly appreciated. Thx

I'd like to clarify further. When the thermostat is turned on, the zone valve open (watching the mechanism) and i hear a click sound but the furnace doesn't fire. The problem persist even after I changed the motor assembly on the zone valve. There are three zone valves and one circulator pump.
Thanks for any help.