iPod synch of iTunes additions/changes


I have experienced repeatedly having just added new music to my iTunes —10.6.3()25)— library or adding a playlist, intent on synching these to my iPod so I can listen, then only to find that the music is not there. I search the iPod after the synch is complete using iTunes while the iPod is still mounted, or more annoyingly after I've gone off with the iPod to do some listening. The only clue I've found is that a not-checked √ tune will not be synched, but it doesn't apply.

What is wrong?