Heatmiser deals with the control side of the heating, not the manifold and plumbing side. When the thermostat calls for heat, you will see the flame icon. If the Heatmiser Wiring Centre responds, you will see zone lights responding. The boiler enable should come on. That means that the control side is working fine.
Therefore as long as the controls are on, it will suggest an issue with the water based underfloor heating, not the controls, which can usually be resolved by a heating engineer or a local plumber.