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Some apps can't be found in the AppCenter in Juno. When I visit their pages on appcenter.elementary.io, such as for Web Watcher, they give the following message:
Note: This app is only available for elementary OS Loki. If you’re on elementary OS Juno, it will not appear in AppCenter. You can reach out to the developer and ask that they update it for Juno.
How much work does it require for the dev to update it for Juno? If it's not a lot of work, then I'll ask the developer to do that.
In theory, not a lot is required for publishing/updating an app for Juno. In most cases, it should just be a case of building and testing the app on Juno against the new library versions and publishing a new release for review.
Sometimes there is a bit more work required if any of the underlying libraries have changed or had deprecations since the last release.
However, for the app you mention specifically, it looks as though there has been very little activity from the developer. There hasn't been any commits since May 2018 and an issue was opened on the Github page asking if it was going to be published for Juno with no response: https://github.com/kjlaw89/webwatcher/issues/24
You may consider forking the application, making some minor improvements and then publishing to Juno yourself.
