Problem: A user that you know to be online appears as offline in Chat.
Check your privacy list settings by selecting Tools > Privacy List within Chat. If you set this privacy list so that someone cannot see you when you are online, you will also not see them when they are online. To remove someone on your list, select the name and then click Remove. Or, you can change the option to Everyone EXCEPT those on this list can see me online
For more information on privacy lists, refer to How can I control how others see me in their contact lists?