There were apps on the PLCnext store which now do not seem to be there. Examples are Teamviewer, Anyviz and Spider control webserver?
App contributors are free to remove any app from PLCnext Store at any time, for any reason.
If you have details for the app developer(s) (e.g. in the documentation that came with the app), you can try contacting them directly for support with an app that they have removed from the Store.