Speed Comping Won't Work

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?

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.

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.

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'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!)