I cannot answer this question with confidence.
There is an article on Wiki about this topic:
http://www.archicadwiki.com/TechNotes/No%20auto-saved%20file%20after%20crash
One other thing that can cause such a behavior is: if the operating system makes an error while trying to create the Autosave files then the autosave files will become damaged and there will be no autosave file.
Or if there is a power failure during that several seconds while the autosave file is being written out onto the hard drive: the autosave file will be damaged and lost.
I am saying this just to tell you that there can be cases when the autosave does not occur for a reason beyond ArchiCAD's control.
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