The right signature can exist in Gmail without being the default for the message you are writing. On a computer, Gmail supports different defaults for new messages and replies. Changing the text of one signature does not necessarily select it for both situations.
Check the defaults, not only the text
Open Settings → See all settings → General and find Signature. Select the signature you intend to edit, then check the default choices for new emails and replies. Save Changes at the bottom of the settings page.
For example, keep a fuller signature for a first message and a short version for an ongoing conversation. Give the saved versions clear names so you can choose them deliberately. You can also use Insert signature, shown as a pen in the compose window, to select a different signature for a particular draft.
Test both message types
Create a fresh unsent message and inspect the footer. Then open a reply draft in an existing conversation and inspect that footer too. The two drafts test different defaults; one successful preview is not enough. Delete or close these test drafts when finished rather than sending them merely to test the setting.
If you use Send mail as with multiple addresses, check the signature for the selected address as well. A signature is message text, not proof of which account is sending: inspect the From field separately. These instructions cover Gmail in a computer browser; mobile signature settings use a different workflow.