I've noticed a couple things when exporting RIF files and then bringing them into fabmep.  1 could be the lengths you have set for your pipe when you set them to a specific system  during process run.  Another could be if there's anything in the Revit file, if it's split there or extra welds.  It could also be the naming that is being used in Revit and the way it's mapped in Fabrication cadmep.  A lot of times I can get maybe 60% from the revit file and fix the drawing in fabmep so it's constructable.