Are you getting any diagnostics via ACS? Any failed attempts or passed authentications? (ie: is the ACS getting anything from the device?)
If not, you probably have a problem in either the configuration of your ACS or the Radius configuration on the client.
If you are, you might be able to figure out the problem from the error message given.
Otherwise, we'd need alot more info as to how everything is configured to help diagnose this any further.