Gas and Oil Home Heating Furnaces - heating not enough heat on first floor

We installed a gas furnace about 5 years about and put a second heat zone in the second floor. The first floor was already zoned and the furnace just needed replaced.
we are not getting enough heat in the first floor. We heat with gas furnace and base board hot water. The second floor gets comfortable but the first floor stays around 58 degrees. The thermostat has been replaced and that is not helping. We were wondering if it could be the circulator pump? And how hard is it to replace? There aren't any numbers on ours or names to find a replacement, could you please help? Thanks.

If the thermostat is on the first floor, the heating system should not satisfy until the room temp matches the setpoint. No ifs ands or buts. If you have it set for 68 does the first floor get to 68? Your post seems to indicate that it doesn't. If not, your problem is in the control circuit. Possibly a heat anticipator setting in the thermostat. What you may find is that when the first floor gets where you want it, the second floor may be cooking.