I was editing my fairly complex timeline and suddenly Page Up/Page Down, which I used to move from clip to clip and use to insert transitions just ceased to work. Page Up takes me to the first clip on the sequence, and Page Down doesn't work at all.
I have rebooted the computer, etc to no effect - is this some kind of setting?
Doh!! I had to go to the 3rd audio track.
CS4 added a few "wrinkles" to some operations that users of other versions have tripped on. You are not the first, and will likely not be the last.
Several things have caused many to scratch their head.