Hi Anand,
What type of tacacs accounting you are looking for?
Command accounting?
From your post it seems that the issue is with one/two NDG, if that is the case then make sure you have those devices configured for tacacs accounting.
aaa accounting exec default start-stop group tacacs+
aaa accounting commands 1 default stop-only group tacacs+
aaa accounting commands 15 default stop-only group tacacs+
Incase you have above commands then get debugs
debug aaa accounting
debug tacacs
This will let you know where the problem lies.
Regards,
~JG
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