Sinking in hole

We have only had this mattress one year and three months - we rotate the mattress monthly as instructed. However, after the first month, the mattress sinks where we lay and does not return to original support. Rotating the mattress does no good. We purchased the king size mattress - there is a hump in the middle, with sinking holes on each side. We are very disappointed with this mattress and hope that the retailer will help us return this mattress to the manufacturer. For a mattress of this price, it should be comfortable and sinking holes should not occur.