TACACS+ Failover

Hello! I recently set up a TACACS+ server for network devices at my company. I set a local user and pass on each device so they could be accessible if TACACS went down. But the local username and pass DO NOT work if TACACS is down. Here is my aaa config.....

aaa new-model aaa authentication login default group tacacs+ local aaa authentication enable default group tacacs+ enable aaa authorization console aaa authorization exec default group tacacs+ local aaa authorization commands 1 default group tacacs+ local aaa authorization commands 15 default group tacacs+ local aaa accounting exec default start-stop group tacacs+ aaa session-id common 

any ideas?