Sank more than 2 inches within 6 months of use

The mattress did hold up good for the first couple of months but as the 3rd month started we started feeling that the mattress was not coming back to its normal height after we would get out of the bed. Called rooms to go and they sent a technician who noted that the mattress did sink but it was less than an inch which was normal with memory foam mattress. But it kept sinking further over the next couple of months and then within 6 months of use it sank more than 2 inches. We are going to replace it with a new mattress. Very bad experience.