Reading throught the "RADIUS Issue Resolution Guide" a common issue I run into is when the radius key doesn't match on the dashboard when compared the client configuration on the RADIUS server. Typically RADIUS servers give no response when this happens. So a packet capture shows a query going to the RADIUS server and no response comes back.
Common causes of this can be from using random strong password generators which include characters that either the Dashboard or the RADUS server can't handle - but still let you save.
I guess the most common cause is human error, from copying and pasting different things.
If you added this it could go in the left hand side of the tree.