Let's say that, based on the dependency to a predecessor, a future (Percent Complete = 0) ASAP task is scheduled by Time Line to start at a particular date. However, if the 'Slip Tasks to As-Of Date' field in the Options, Calculations form is checked, the As-Of Date is the EARLIEST that Time Line will schedule any future task to start.
If this is the reason that a task has been delayed, one of a few things can be done:
uncheck Slip Tasks to As-Of Date. This will cause all future tasks to slide backwards in time, back to either the schedule start date, or to a predecessor (whichever is later);
change the Percent Complete in the Task Info Box to anything greater than 0, and enter the correct start date;
set the task type to one of the 'Fixed' types, and enter the correct start and/or end dates.