Symptoms

Task SLA records do not show value for business elapsed time.

Cause

Schedule attached to SLA definition does not have schedule entries. 

Resolution

When you open the SLA definition, in the top of the record you will see the warning message: 'Warning: There are no active entries in the Schedule, or its Child Schedules'. Looking at the schedule definition it can be seen that there are not any schedule entries for this schedule in the 'Schedule Entries' related list.