ArcGIS Server Manager does not generate a server statistic report

[#1]

We're tracking the service usage and response time by monthly.

But some metrics related with response time are not generated.

This issue only reproduced with network services.

→ Total requests are collected normally.

→ Average Response time with same services are not collected.

→ Also, Maximum Response Time are not collected.

→ When I generated usage report using server administration page, "N/A" value are returned for Average Response Time.

I already checked below article and applied the guideline provided. But, the response time still not collected.

https://support.esri.com/en/technical-article/000014455

Is there anyone who have an idea about this issue?

*p.s.  The version of software is 10.8.1.