Sagging Bed

We bought the Hotel bed from the Bedshop in August 2014.In less then year and our bed is sagging!!! It is suppose to handle 140kg per person but its not! We are far below the weight limt! As soon as my husband and I lay on the bed,we role towards the middle! We contacted the Bedshop and told them about the problem 18 May 2015.A week later they sent someone to have a look at the bed,but he couldn't find anything wrong after \testing"the bed rolling around on it!? I told him he alone is too light"