Element and part levels display

Can anyone explain why elements placed on a level and Building Designer parts and families do not display in the same way?

i.e. for example, linear / shape elements are placed on 'concrete upstand' level then converted to parts / forms on the same level - all other levels switched off, and only the simple elements display, whilst the parts don't ? This despite the form having the same level attributes as the simple elements..... 

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