I'm with dcoffset. Start by making sure all tracks you wish to comp as a set, are in an active group. Next, make sure that playlists are indeed being selected correctly(if not, you may need to disable the group and re-order playlists on some tracks). I also second his comment on adding a new playlist for the "comp" track(I usually name that "trackname.comp" or "trackname.final" without the quotes).
Further advice:
1-when auditioning playlists, hit STOP when changing which lane you are selecting(with the lane SOLO button) because if you don't hit STOP, what you hear might be the last playlist, at least for a few seconds(IOW, switching is not always instant). Hitting STOP and PLAY means that what you hear IS what you expect to hear.
2-for future enhancement of the process, name your original track as "trackname.01"(without the quotes). Doing this will make each newly added playlist automatically named with the next number(so, trackname.02, trackname.03, etc). That way, you know that .01 is the first take, .02 is take 2, .03 is take 3. This helps a lot in any tracking, but helps a ton when doing multiple takes on multiple tracks
