Locating lost refund

I did my taxes for 2019 2020. Was issued a refund, and was sent the refund by mail. After waiting for months I realized that it never came so I tried contacting IRS, after several blocked,disconnected, and failed attempts to reach a parson I finally was able to determine that my return had been granted and that it was sent to my then address. I was given all contact info and linked numbers as well as extensions for talking to someone. Apparently my return was never delivered and was stated as being "undeliverable" whatever that means. I assume the mail man was unable to see or get to the mail box I don't know. But I never received my refund and have spent literally years now trying to recover my lost return I've tried contacting ,writing,e-mailing,calling and still cannot find out what to do to have it sent to me or anyone I can talk to to have this remedied please .