My employee's compensation changed but it's not reflecting when I run payroll. Why did it not sync over?

If the employee's compensation was updated when the pay cycle was active (i.e. - you already started processing payroll) then their prior compensation record will have already been pulled into the pay cycle. 

You can easily update the employee's compensation in Step One of payroll. Click Delete next to the employee's name to remove them from the pay cycle. Next, re-add them to the cycle from the Employee List drop-down menu. Their updated compensation will now be reflected in the pay cycle.