Welcome to the forum.
By "render," do you mean creating preview files, for the smoothest playback, by hitting the Enter/Return key?
If so, there are a couple of reasons that a Timeline might not Render:
- The Clips on the Timeline do not require Rendering for smoothest playback - in that case, there will either be no colored line, or a green line above the Clips in the Timeline, in Expert View.
- The WAB (Work Area Bar) is not set for the full Timeline's Duration, i.e. not covering all Clips.
For more info on Rendering for smoothest playback, plus some links to the WAB, see this Tips & Tricks article: http://forums.adobe.com/message/3484339#3484339
Good luck, and let us know if that helps.
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