Sorry to hear that you are having issues. Your videos don't quite give us all the information that we would need to diagnose, however one of the more common issues has to do with CORS. If the Web Viewer or the location where your PDF is stored does not support Cross-Origin Resource Sharing (CORS), then it may have an issue loading. This is 100% on the browser-side, we can't do anything within the library because it is a client-side library.
There could be other issues, but it would require us seeing in more detail the code being used to embed within your app to determine other indicators.