Hans it looks like a bug that happens from time to time with complex ceilings.
Actually, Sweet Home 3D tries to compute the intersection between a ceiling and the floor at the upper level, and sometimes the program isn't able to display this shape if it's too complex. In your case, it can be fixed easily by moving the top left corner of the largest room you draw at level 0. You'll see that as soon as you place this point at walls corner (it's not at that place in the version you shared), the ceiling will appear.
In one word, complex design requires more precision from the designer.
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Emmanuel Puybaret, Sweet Home 3D creator
