Changing Task Constraints
If you manually enter a start date or an end date for a task, Project changes the constraint type for that task to ‘Start No Earlier Than’ or ‘Finish No Earlier Than’.
These constraint types force the task to start or end on the specified date regardless of subsequent changes that would automatically schedule the task’s place in the overall project plan. This behaviour confuses many new users to Project.
Change the constraint type back to the default, ‘As Soon As Possible’ and Project will adjust the dates for the task optimally in relation to the project plan as a whole.
Graham Perry