Floor slabs in Revit export in same level.

I got an IFCexport from Revit and hoped to turn off one floor slab to expose the storey below with walls an some details.

Unfortunately all  floor slabs in a Revit model is exported to same ustn-level.

Anyone know of some trick I can tip the Revit guys so the export puts the different floor slabs in different ustn-levels.

(thought I posted this a few hours ago - if the appearance is delayed - sorry for taking bandwidth)

Is my wording above correct  'in' a ustn-level but  'on' a level in a building?