Calming 3000 Pocket Sprung Mattress

The mattress is comfortable, however after 2 weeks I'm disappointed that there is already a noticable ridge down the middle and 2 dips either side where we lie. The mattress is a non turn and is recommended to rotate weekly which we did. I got in touch with customer services who asked for details and pictures which I provided only to be told that it's just settling and it will only be returnable under warranty if the dips are 5cms or more. Well judging by how quickly it's dipped 1cm then that probably won't take long. Unsure why the issue is happening, I can only assume its cheap memory foam on top of the springs. But I'm sure we will be returning under warranty soon which is disappointing.