hi ..
I can not log on there .. it seems.
I can log onto ArcGIS.com, but not this site .. weird .. is this a special account of yours ?
Concerning visibility :
Check the visibility buttons in the web viewer when open.
Maybe post also a screenshot of the web scene since I cannot access this one.
Otherwise, try to track the issue down one by one, e.g. only exporting 1 building, then 2, then 3, ..
Ok ?
M.