Do you have Ops (+o) on that channel or IRCops on the network when issuing that command?
If not, then the reason you can't do it now is due to a more restrictive checking as to who does and does not have authority to set those modes.
If I recall correctly, and I can't find the actual post on this right now, simply put you have to set a channel user mode you have to have the next level or higher. Eg: To set Half-ops you have to have Ops or higher. To set Voice you have to have Half-Ops or higher.