Mattress rolls away from middle on sprung slatted base

We bought a bed from Dunelm Mill (I know they are not bed specialists), it’s has sprung slats.
We put a memory foam mattress on it (thin and cheap mattress), we have replaced the bed once and the mattress twice ( so three of the same mattresses and two of the same bed )the same problem has been there from the start till now.
The bed is raised up at the middle and we don’t have a clue why. Is this mattress may be just too thin and cheap, if we go for a high-end heavy mattress would this solve the problem, don’t want to replace the actual bed because of the furniture in the bedroom matches ... any help or info would be great, thanks very much, Mike Letford