Ensure that accounting is configured on the NAS (aaa accounting network ...) and that is runs at least IOS 11.0(3). You can test if the accounting messages are sent by "debug aaa accounting" on the NAS.
In the case of a UNIX TACACS+ daemon, check that the accounting file is writable by the daemon user. Otherwise the following can happen: The daemon starts as root, opens the accounting file, changes its userid, re-opens the accounting file but fails because it is only writable by root.