Hi Durga, 

This exception is raised when unallocated activities are expected to exceed their SLA deadline as a result of not being allocated - i.e. if the SLA deadline is before the end of the scheduling window (including any appointment window). This includes 'do not schedule' activities as well. The 'do not schedule' activities will not show by default in the list of Unplanned Activities. This is because they are at status do not schedule, and so it is expected (and so not an exception) that they are unallocated. However, they will show in the list of unplanned activities in jeopardy if they are missing their SLA, since this is considered an exception. The activation setting  of the schedule exception (in Administration-->Organization → Scheduling exception screen) holds the time in seconds that the activity must have missed its primary SLA by for the exception to be raised, default 0.

To determine why an activity is not being allocated, you can submit a new explanation request via the Scheduling → Explanations screen. This will help identify the reason why the DSE did not allocate the activity. Alternatively, you can use the Compare screen to analyze the activity's constraints against those of a potential resource you believe should be allocated to the activity. This can help clarify why the allocation was not made as well. However, if there are a large number of unallocated activities, these approaches may be time-consuming.