Also remember F12 is for range chat. If no one is physically near you, they are out of range and won't hear you. Open your radio window, and see if there is another name under the purple word "range". If not, then no one is close enough to hear you. If there is another name, and they still dont reply, they may not have a mike.
Another trick I learned is if you open the radio window, and the check, and then uncheck the AFK box, it will allow you to keep the window open and still hit keys without putting it in the text bar. (Nothing more frustrating than seeing TTTTTTTTTTTTTTTTTTTTTTT fill the text bar). If you do want to type something you will have to click your mouse in the text window to get the flashing cursor back.
Chat does work regardless of the window being open or closed if tuned correctly, or if using range someone is near enough to hear. I open and close the window occasionally just to see who is in range, or tuned to the T channel I'm on.
Hope my 2 cents help.