3 Fautly mattresses in one year!!

Bought Serta Carina matress through Dial-a-Bed at the end of 2013. 1st matress that arrived had to be sent straight back due to a tear in the side through which the padding was peeling out. Then had to wait for more than a month for a new one since Dial -a-Bed were out of stock and had to wait for a new mattress from Serta. Eventually replacement arrived. After 6 months (still under warranty) the middle of the mattress had sagged. Serta replaced the mattress since it was still under their warranty. Serta's response time was not too bad (???) and I had a new Carina within about 6 weeks. Mattress was delivered on 4June 2014. By November the same problem had occurred: the mattress had sagged. I contacted Dial-a-Bed in November 2014 to have them alert Serta that the new mattress had also sagged. Dial-a-Bed contacted Serta in November to let them know about the problem (I have confirmed this with Dial-a-Bed). It is now February 2015 and I STILL have not been contacted by Serta. Bad product, bad service and I'm out R6000 which I paid for a mattress that now causes back ache. EISH...