Nested Movieclips issue

I’m pretty new to actionscript, and am trying to play a nested movieclip by clicking on a button. I have an instance name for the nested moviclip, and this is the code I atatched to the button:

on (release) {
_root.conference.gotoAndStop(35);
}

Nothing happens when I click the button with that code. However, if I take out the instance name, the button will play the frame specified:

on (release) {
_root.gotoAndStop(220);
}

Can anyone suggest what I’m doing wrong?

Thanks