I used the speed comping trick on one song and it worked great.  Went to a different song, divided the clips, and then only one clip will play when I do the Shift+Enter method.  I can move around via the arrow keys, but some clips are silent (even though they're not muted) and some actually double another clip.  What am I doing wrong?

There is the lane mute, but there is also clip mute that can be triggered with the k key on the keyboard.

gswitz... It's more complicated than that.  None of the lanes are muted (M).  All clips are solid (not grayed out, i.e., muted with "k").  When I speed comp (shift+space), only two of four clips will play when I move to them via up/down arrow keys.  The silent two are apparently muted, with no obvious indication as such.  
 
In another mind-bender, I move to the next section of the song with four more clips and this time the clips in DIFFERENT lanes are muted and only one clip will play regardless of which lane I move to.  Maddening.
 
Could this be related to how I recorded the clips in the first place?  It was one of my first attempts with X3 Producer and I was experimenting with the Sound on Sound vs. Comping vs. Overwrite options in Preferences|Project|Record.  I can't see how that really would make a difference, as I have four takes in four lanes, none of which are overwritten.

Just a thought...

Look to see if somehow a clip volume envelope got changed to zero?

I seem to remember a shortcut that uses something like shift-enter to directly access this...

Thanks, Keni, but no luck.  They're all at 0 dB.  Another clue:  I can hear each clip when comping the old-fashioned way by clicking on each clip and muting (or unmuting), as desired, while looping.  As soon as I invoke the shift+enter speed comping, some of the clips go silent with no hope of unmuting them.  I've tried this with and without the Dim Solo feature and get the same result.

I couldn't wrap my head around speed comping until I followed the tutorial and subsequently, followed those same steps. You might find this article useful.

Thanks for the tip, Anderton.  I had already looked at that document and the video from Cakewalk.  I think I understand how the speed comping is supposed to work.  In fact, I successfully employed it on another song.  This song, however, has mysterious, random clip mutes, even though the clips appear unmuted.  Look at the image below.  In the middle section, Record 2 and 4 are audible, but Record 1 and 3 are silent.  The next section over, all four Record clips are audible.  What gives?  As mentioned before, all clip volume envelopes are at 0.0 dB and no lane is muted.

 
 

I don't think you are doing anything wrong. I've experienced similar issues with the speed-comping method you use, so I always just click the lower half of each clip in the section when I want to monitor each take quickly (btw, when doing so, I recommend locking the positions of all clips in the lanes, since it can prevent you from accidentally/inadvertantly move a clip a bit when the Snap to Grid is disabled, and you can still adjust the split points when only the clip positions are locked).
 
Take lanes show a number of unexpected behaviors like this. I guess the feature wasn't fully tested before the release.

I'm getting closer.  When I'm in speed comp mode, a problematic silent clip will become audible if click on its ID bar at the top of the clip.  If I just move around using keyboard arrows, the problem is intermittent.  Oh, well.  It's a work-around.  Sheesh.  (Thanks icontakt!)

Clicking on the lower portion (below the clip title/grab bar) will immediately solo that single clip in its time position.

Clicking on the grab bar and then using mute/unmute (k) will then allow this to be over-ruled so that more than one Click-time can be hear...

Without ever using promote command...

Interesting pairing 1/3 vs 2/4 as if it's a stereo issue?