Service no Service

I have a 2 Suntank units installed in late 2012 and has been servicing them clockwork annually as per the T&C of the units and the last service this year was done and a week after the 1 unit stopped heating, the service team was deployed several times and every time a few days after they service again and state they have fixed the issue the unit stop heating again. I have been sending numerous emails begging for this matter to be resolved and little to no response and every time the excuse is they struggle to assist on a Tuesday or Friday as the available dates supp****. This matter is been on-going since July 17. I am seriously disappointed in the service and also for not having hot water.