Hoping that "Little Snakey" you have had your issue resolved. Other issues with saving here is a idea.
As far as the document getting saved, if this happens with a file saved either "on the network" (as in another computer, not their local physical machine), or if they are saving the files on a usb drive.
The reason for this is because if the computer drops the connection with this remote computer and/or usb drive, then Flexi lost communication with the file even if the communication is recovered.
A good test is check whether they can save files directly into their desktop, and if their temp files in Flexi are also pointing to a remote computer.