New bed/ mattress sinking in the middle

[PickledWilly]

We bought a new bed from Next in the sale which was delivered last weekend. It has wooden flexi slats on the base. Unfortunately our mattress (big heavy Hypnose one) is sinking in the middle now on the new bed. I am about to buy a new mattress, but am worried this will happen with the new mattress too! Has this happened to anyone and do you have any mattress suggestions?!

Are the slats on the base the right way up? I used to work for a bed manufacturer and the amount of times this happened was unreal. Are they sprung or solid slats? Sprung ones are always bowed, hence checking they're the right way up.

Thanks for replying! They are sprung slats and they are bowed at the top like a rainbow (is that the right way up? I think it is)

as you have a heavy mattress you could put a thin piece of chipboard on top of the slats this would stop them sinking in the middle this is what i did with my last bed, would be cheaper than buying a new thinner mattress