This happens when your old account on a Bluesky server is not turned off properly and those people have their accounts on the same Bluesky server.
What happens is that the Bluesky server looks for your account locally first before checking the whole network. It finds the old account, which is no longer active, and there is nothing to reply to.
The good news is there is an easy fix: go to PDS MOOver Turn Off, enter your old account details and it’s done.