Can't make a clip in FinalCutPro X longer
The red line is shown when there is no more source media left. You've dragged the end of the clip all the way to the end of the source file, so there's no more media to be added. Imagine a video 5 seconds long, of which you've added the first 4 seconds to the timeline, then you try to drag the end of the clip in the timeline beyond 5 seconds — you can't, because there's no more source media in the clip.

The end of the timeline when viewing your compound clip is greyed out because this section is beyond the end of the compound clip range in the containing timeline. The main timeline which contains the compound clip is only showing a portion of the compound clip, and the video in the compound clip beyond the greyed out area will not be played. To play this section, drag the end of the compound clip in the main timeline until the red line is shown.

To quickly show/hide the retime editor for any selected clips, press cmd-R.