Newbie question: mc on separate timeline?

I have a mc with a tweend animation that’s 18 frames long. This mc is placed into the main timeline on a keyframe, but the same mc won’t play thru unless I have it existing in the main timeline for the number of frames (18) that is required to play the mc animation. I thought that you just place a mc on the main timeline and it’ll play it’s own timeline entirely without being ‘tied’ to the main timeline? Am I missing something here?! This is simple but it’s not working somehow!

thanks