Unsatisfied customer

I bought a bed at Lewis store in Eldorado Park. It was not even 3 months when the mattress was sagging on the one side. I went to the shop to put in a complaint but nothing was done. The day when I settled the account I ask them again about the mattress and they just said that they will get back to me .I sent the manager numerous emails but up until now nothing has happen. He even ignores my emails. I even sent the sales lady WhatsApp massagers but they ignoring me.