I think that the main reason is:-
The size parameters linked to the connectors have their Discipline set to either Piping or HVAC and their type of parameter set to either Pipe Size or Duct Size (respectively).
Try making a rectangular box (as a pretend Attenuator) with L, W, H parameters and add parameters to the dimensions. However rather than making the W & H parameters normal length types, make them as listed above. Don't forget to make the family category "Duct Accessory" and the Part Type "Breaks Into". That should work fine.
Another aspect which may influence this is what Part Type is selected in the Family Category.
The text file used by some RME families is a catalogue and is just another way of containing all the values used by the parameters to determine the sizes in the families and their types. I allows you to only load a selected number of family types rather than loading ecery type contained in a family if the dimensions use look-up tables, etc.
