My brand new hot water baseboard heater doesn't give off much heat
There are a number of possibilities:


Air in the system. After installation or repair, it's almost inevitable that some air gets into a hydronic heating system, and there are valves on each baseboard convector and at the pressure tank to remove air from the system. Though automatic bleed valves will slowly remove air, you can do so manually. Be prepared for some water overflow with rags and sponges, since fitting a pan underneath a convector is difficult.
Look for balance valves in the basement which shunt water between parallel routes, and check that the valve to the living room is fully open.
There may be flow valves at the end of the baseboard unit... depending on design.
During installation, a pipe was damaged or filled with solder.


N.B. Since you (or someone) paid for this work, it should be covered by warranty. Notify the installer in writing of the issue and have them fix it.