MG ZR no hot air coming into car

hot air as stopped coming into the car just reacently ive had a new thermostat and the system as been flushed as well has the heater matrix box there is no problem with the head gaskit as the car runs at its normal tempreture and shows no sign of over heating the only thing that mechanics can tell me is that there is no hot water getting to pipes that lead to heater matrix box but if systems been flushed cant see pipes been blocked what can it be it has to be something any ideas