In Gmail on a computer, go to Settings > See all settings > General > Signature. Choose the signature for new messages and for replies and forwards, then click Save Changes. If the signature is missing only when you view a sent conversation, expand the three dots below the message first.
01Test a new message and a reply separately
Section linkAfter saving the defaults, open a fresh Compose window and check the footer. Then start a reply and check that footer separately. If you can add the signature with the pen icon in the compose toolbar, the signature exists. Recheck the default for the type of message where it was missing.
02Check the sending address
Section linkIf you use Send mail as, look at the From address in the message with the problem. Go back to Signature settings, choose that address in the drop-down, and check the signature assigned to it. If the address is missing, look in Accounts and Import under Send mail as. Save the changes and test with a fresh message from that address.
03Look below the sent message
Section linkOpen the exact message in Sent and expand Show trimmed content if you see the three dots below the body. If the signature text is visible and only its image is broken, check that the image still exists. For a logo stored on Drive, use a file you are allowed to share with recipients or upload the logo directly if your account offers that option.