Hi Sindy,
This was posted to the CA Automation community so I am moving it to the CA Release Automation community. In regards to your question, I don't have an answer. I wonder if it stores any historical data for X amount of time and maybe it is just a matter of time before it is removed. Otherwise, I'm not sure if recycling the NAC would help but have you tried? If neither of these things help and there are no other answers offered then you can open a support issue for us to look into it further.
Kind regards,
Gregg