SSAS Multidimentional Drillthrough with Many-to-Many Relationships
If your drillthrough command includes a dimension attribute from a many-to-many dimension then it behaves as described: showing a row zero to many times. 

The easiest solution is to create a drillthrough action and mark that action as the default drillthrough action and specify the columns you want to include in the drillthrough results. If you don't include the many-to-many dimensions then it will stop skipping or duplicating rows.