The problem is due to the diameter of the spring inside the mattress. It is common for the manufacturer to cut cost by using less or smaller diameter spring for its mattress. Hence the mattress would only be good for few years.

Go and try out pocket spring + memory form mattress. You might like it I have got 2 mattresses in the past few years from a local manufacturer, and the quality has been This company exporting most of their mattresses to Japan. Locally they are no so popular as they don't advertise on newpaper and etc. Do check out Simoni mattresses if you are shopping for a new bed.

P.S. I'm not related to the company, nor do I get any $$$ from them. Just sharing a good product here

Normally the shops would have a sample for all the models which would show you what is inside the mattress. So far I only have seen few pocket spring with memory form mattress out there.

Alternative you could get a pocket spring mattress and look for seperate piece of memory form (like a mattress protector) to make up the combination. Otherwise just ask the dealer to contact the factory to request for your needs. As some models may be available for their export market sales, but not available locally.