Great at first but now has indentations

First 6 months great but you can now tell where we sleep. We bought a bed with slats and I did ask the salesman if this was ok and he said it was. Just read the terms and conditions of the warranty and realise its easy for them to get out of it. I went for this mattress as I was concerned about overheating and ok it has been good for this. But I now always wake up with shoulder and muscular back pain. Its difficult to roll over as there is an impression in the mattress. I sometimes even wake up during the night. Never had any problems with my last sprung mattress; just got rid of it due to its age.