Did you check your tablet against the VPN apps requirements to make sure it will work on that tablet as well as work with your version of Android? If not, that is what I would do first.
If you then can't find the above info, try contacting the apps creators. See if it is a known issue and one they are working on, or is an issue they need to fix.
Then check the "Settings" in the app and see if there is anything not set correctly.
Should none of that work, then I would suggest trying a different VPN app.