concurrent sessions per user

I have problems in limiting the no. of concurrent sessions that a user can use to connect to the database.
Using enterprise manager, when i reduce the 'concurrent sessions' parameter, it is not effective. I then tried to create another profile having just one as concurrent sessions. But it allows to have more than one. What may be the problem?
Should the database need to restart before the changes take effect?
thanks
manjunath

Be sure you have RESOURCE_LIMIT set to TRUE in the init.ora

Thanks a lot ramon,
it was set to 'false'. I will correct the mistake.
manjunath