Retrieve hole feature data from an ISK part file using VBAPI in C#

We normally retrieve hole features from part files using Solid.ListFeaturesByType(false, (int)EpfcFeatureType.EpfcFEATTYPE_HOLE). However, this code doesn’t work for certain ISK part files and returns a null group value. Please explain, why the part file is returning a null group value?