Do you use more than one email address? Did you reply from a different email address than the one you originally used? Email replies require that you use the same email address when replying.
Did your email software properly use the Reply-To: email address when replying? Unfortunately, some email software incorrectly sends replies to the From: address, which won’t work.
Was the Message-ID header in the email modified? The Message-ID must be consistent and unchanged.