Sometimes you will get chat rooms with a lot of nicks in the room, but the people using those nicks aren't actually there.
If you type //msg $me <any message here>
you should get a pm window with your nick in it and the message you posted. If you get that, then your messages are being received by the server/network.
If it seems like people still can't see what you're saying, type //echo -a $chan($chan).mode
If the letter m is in the response, then the room is moderated, which means that an op has to give you a voice, as indicated by the + character at the front of your nick.
If these don't work, try typing /!remote off
then trying again.
If you still have no luck, post back here and we might have some other ideas.