I am using MS Project Standard 2003 SP3 and have experience instances in which the automatic calculation of work effort fails to work when I change duration. Has anyone else experienced this and know why it happens and how to prevent it?

I created by project schedule with if effort driven and began to modify some resources. As expected, the modification to the resources updates the durations for the tasks, so I change duration back (since project is fixed duration on tasks) and the work updates as expected, so I proceed down the remaining 600 lines. Upon reviewing the resource usage I notice some are way off and find that not all the work efforts updated. In order to 'game' project I enter a very large effort (555) and then enter the right duration and magically the effort calculates properly.

Needless to say I would rather not go through all 600+ lines again in the future and would prefer to not have to check every line item I change to ensure project calculates as expected.

I would very much appreciate any feedback or words of advice. Thanks

John