[CentOS] I added my user to a group last week, and now the user isn't in the group

Hello everyone,

I added my user account to a group called "www-pub" for easier
manipulations on web files on my server. And today I found that my account
was not in the group "www-pub" any longer.....
What I did to add my account to the group is like "sudo usermod -a -G
www-pub myaccount". Is there anything wrong?

Thanks for any suggestions.

Sincerely,
Xuwen