Is it all underfloor, or is there Rads as well. The clue was in the fact it worked the other day, was that coz some of the other zones had shut off on the stats giving a bit more push to the remainder. With as many zones as you have, it may need balancing, is there a trimmer valve on each circuit? That's what the other valve on a Radiator is for, not just removal for decorating. If you have,small or short run zones, you need to cut the flow back to put more flow through the larger, longer run Zones. With Radiator systems the rule was to have even heat through the Rad, but with 11 degree drop from flow to return, we always used to guess it, then TRV's came along and made a nonsense of it all as they trim flow when working, everything usually got left wide open. It's not heating, coz it's not getting enough flow, it could be a kink in the pipework or a faulty component. Shut down all the other zones and see what happens.