Return declined - mediocre service

Mediocre service I bought a mattress at sleep master in January and it was delivered in March. Upon delivery, the delivery guys put the bed on the base and then we realised that the length is short. They then said I must contact the store as they are just doing delivery won't get involved with wrong orders. I then went to the Sleepmaster shop where I bought the mattress and I was told to keep it and they would contact me when they got a buyer for the mattress. I went again in May to enquire I was then given a phone number to call, where I logged my query with the sleep master call centre agent. I made a follow up in June and I was told that the call is still active. I followed up again today to be told that the exchange is declined