DLJester
When in ToD (this may happen other places as well, but I haven't tested), if you die from sitting in one of the auto-death chairs, you will no longer be able to follow your group. You can click on the leader and select follow, but your toon will not follow the group around the current screen nor will they move to additional screens with the group. Leaving the dungeon does not change the behavior, and neither does regrouping. The only thing that resets this is relogging.
This reproduces for me 100% of the time in ToD with the chair on the third level. It happens in other places in the dungeon as well, but since the deaths are random with other chairs/buttons its hard to test accurately.
If I had to take a guess, I'd say the issue is likely that the character is stuck in some internal sitting state which disables follow. If this is the case, one possible fix would be to force the toon to stand before being killed; similar to the teleport from sitting behavior.