You've mentioned you haven't got the SuperSU app, have you got Superuser app on there at all?
The only thing I can suggest is possible performing a factory reset on the device. This will take it back to how it was when it came out of the box however by doing this you will be wiping your tablet so make sure anything you want / need is backed up first.
Let me know how it goes.
Kath_F Forum Team
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