The resolution was down to a configuration change in the Job Cost Formula used by the Configuration, this was changed to use the SUM_TASKS_LP cost variable rather than the standard SUM_TASKS one.
If this does not resolve the issue, it may be that all tasks are not correctly assigned. Are there tasks in the Job which are not pre-processed? If there are, this could be a cause and may be fixed with a configuration update. Please raise a case with RWS for investigation in these instances.