Approval/ invitation letter from HO not arrived

Hi. I applied in February 2021 for British citizenship. Did the biometrics in March 2021.
Heard nothing forme the HO since. My local authority contacted me by email offering me a date for my citizenship ceremony. I contacted them to say I hadn’t received my approval / invitation letter from the Home Office. They also said they had my certificate from the HO ( received in June 2021) but to wait for the letter from HO.
It’s 3 months since my local authority has received the certificate from HO and I still haven’t received the letter ( Sept 2021).
Could someone advise me as to what to do. HO don’t take phone calls and I haven’t had any replies to my emails. I seem to be going round in circles. Thank you

Have you also checked your email spam folder from the last 4 months in case home office sent the invitation letter by email?did you give the the correct email address when you made the application?Have you asked your local authority how long they will keep it for before its sent back to home office?