The liquid in your radiant heat system is circulated by a pump or pumps from your boiler. The flow of hot water or liquid to each room must be balanced according to the heat-loss from insulation or lack of it, amount of glass or doors in the room, sunlight, etc., in order to keep the room comfortable. Therefore the temperature of the water and the flow to each room of this hot liquid must balance or match with the need for heat (heat loss).
There is usually a valve or shut-off faucet type of control to adjust the water flow to zones in the house. This valve might be a manifold with several faucet-like handles in a bank, or the valve or valves may be positioned around the house in various rooms. Look for the valve which controls that room and increase the hot water flow to the cold room. Sometimes this increase is from closing or restricting some flow to the other parts of the house to force more flow into the “cold” room.
This balancing process may take quite some time to achieve. I suggest you have a radiant heat specialist visit your house, identify the valves, and teach you about your system. It is likely time to have the system serviced and the imbalance may require flushing or cleaning pipes in the system and replacing it with fresh liquid or water. I think it is time for some instruction and help. Your neighbors can usually help with this knowledge and reputable companies familiar with radiant heat systems.