Have you checked the logs for that app? They are located in a directory set in your shiny-server.conf file. The default location is /var/log/shiny-server (at least on centos 7, I'm not sure if this is different on Ubuntu). cd to that folder, and look for log files corresponding to the app you're having trouble with. These log files can help diagnose issues with your app, and it 's always a good place to start troubleshooting when your app won't run on the server.
I also think andresrcs raises a good point. If you installed it running as your own user, it wouldn't be in a package library that the shiny user has access to. It needs to be installed either as root (for a system-wide installation), or as the user shiny.