Disappointed at the quality of Strand mattress

I bought a single bed set from SleepKing. It was a cloud nine strand mattress. I bought the bed on the 20th of July 2025 for my daughter. She only used the bed recently about 2 weeks and the centre of the mattress is sinking. I have complained on the 18th October 2025 to the store where I purchase the bed. I also went online and made a complaint with cloudnine. I have not received any response yet from Cloud nine.