well it dependrs how thin they are.
if they are thinner than the nozzle width - then they won't appear in the sliced model.
If you can't edit the model then you could change the nozzle for a smaller diameter. 20 0.3 or 0.35 instead of 0.4

Had a look through flashprint and I can't see any option for nozzle diameter. The closest I can find is 'path width'.

Adjust that and resclie the model - if the window and doors appear - then that's what's causing it.