Retrench, with a account arrears letter

Hi, I was retrench in September 2020 due to covid-19, I put in my claim, they cover 12 months, so my claim ended in September 2021. They said that they try to get hold of me after the claim was done, but they contact a number that is not available, I received a account arrears letter in the post after 3 months, received the letter 18 January 2022, and the letter was made out on the 4 January 2022. My question is why couldn't they send a letter to me in October 2021 or November 2021to say they could get hold of me and my claim is done in September 2021, after 3 months, now I get a letter to say my account is arrears, I could've sort it out that time already and I am still unemployed, now they want me to pay the account. I am worried and stress.