I guess you used the file-publishing feature. If you published a file and then modified the file, a new file ID was created. The old publish URL is invalidated.
To solve the problem, you just need to use the Folder-Publishing feature. Instead of publishing a file, you can publish the parent folder of the file, then use the following Publish URL to link to the files in the published folder:
For more detailed info, please read Chapter 5 of the service manual.
You can also use our Static Web Hosting feature to publish files. (It might be slightly easier. You don't have to host a website. Please watch the video: https://youtu.be/bPw4N1VDLKY)