It is possible there are other predecessors driving the task so your
"external" predecessor is not currently in play. In P2007, you can use the
Task Drivers pane to see what is going on. In P2003 (and earlier),
Window/Split... In the bottom pane, you will see predecessors, double click
on them one at a time to see the finsih dates and learn which one is driving
you.
Just as a note: Looking at the predecessors for external tasks, you will see
the complete path to the project file and its specific line number.
If you have only one predecessor and it did not move and your successor is
ASAP, try pressing F9 to recalculate.
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Jim Aksel, MVP
Check out my blog for more information:
http://www.msprojectblog.com