The issue of 'cuttable' families came up again today. It's often a confusing topic of discussion and trying to explain why some categories are different to others is never a convincing argument. I posted an idea about it at the Autodesk 'Revit Ideas' Forum recently.
The purpose of this post though, was to link you to a number of blog posts Luke Johnson from 'What Revit Wants' has written about cuttable vs non-cuttable families and expected behaviour. It may help you troubleshoot problems on the subject. Luke has also prepared an open source google spreadsheet capturing the unique properties of all the categories.
It can all be found HERE.