Have you tried to increase the boiler capacity in addition to the zone system capacities?

On oversizing zone systems for test-cases in your energy model, it can sometimes be good to try something extreme, like 200% oversizing. Depending on the space types you selected for the various rooms served by the thermal zone and terminal unit, there could be different loads (or lack of internal loads), than what the engineer actually assumed when they designed the system.

Lastly, check the airflows of fresh air match the design intention for each zone. Again, if the design assumed less ventilation and different space uses than what you assumed for specific rooms, the model could be over ventilating and resulting in higher heating needs than what the HVAC system was designed for.