However, one or more users could not be converted to Claims

I followed this article:

http://technet.microsoft.com/en-us/library/gg251985(v=office.15).aspx

section "Convert SharePoint 2010 Products classic-mode web applications to SharePoint 2013 claims-based web applications" to convert our SP2010 site to SP2013 CLAIMS mode.

After I run "Convert-SPWebApplication –Identity <yourWebAppUrl> –To Claims -RetainPermissions" and completed. It prompt for warning:

WARNING: The conversion of web application and most of the users to Claims mode
 is completed. However, one or more users could not be converted to Claims
mode. Refer to the ULS logs for the details. For the troubleshooting tips refer
 to the http://go.microsoft.com/fwlink/?LinkID=236943 article.

I tested login with a few users and seems good.

1. Is the warning valid?
2. How can I find the related log? It even not providing the correlection ID.

Thank you!