Whenever I apply a slow motion or fast motion, After effects does not lengthen or shorten the scene. How do I get After effects to lengthen or shorten the scene whenever I apply a time adjustment?
What command are you using to change the speed? There are several ways to lengthen or shorten the scene plus numerous plug-ins. If you use Layer > Time > Time Stretch, the layer should be stretched to whatever you used as a stretch percentage. You may have to lengthen the comp to actually see the stretched footage.