Automations don't remember the records they have processed. By default, they will process any records where the output field is empty and skip any records that are filled already.
To reprocess records and overwrite existing data in an output field you would need to set the automation to overwrite existing data or manually clear the output field before running the automation.
Example
If you're using the Convert URLs to Attachments on Airtable automation, you'll notice that -by default- it's not redownloading the updated file from your URL. If you're trying to re-upload the attachments when the source URL changes, you'd need to make sure the output field is empty or set the automation to overwrite existing data.