There is an OLAP Cube that contains one or more partitioned measure groups. The "# Historic Partitions Updated" property on the measure group is set to some number e.g. 2
When the cube is updated, either interactively or via the Scheduler, all dimensions are processed but only the 2 most recent measure group partitions are processed. All other partitions are left in an unprocessed state.
The most likely cause for this issue is that one or more dimensions have the Processing Method property set to "Full process". This causes the dimension to be fully processed each time the cube is processed, which in turn puts all associated measure group partitions in an unprocessed state. Since the measure group properties have the # Historic Partitions Updated set to a number (e.g. 2), only those partitions are being updated, the others are left in the unprocessed state.
If using partitioned measure groups, ensure that the Processing Method property is set to "Default Process" on ALL associated OLAP Dimensions.