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.