Switch to the coder view and from the demos menu, select “csvFromPsydat.py”.
To be honest, the most common cause of this is the experimenter accidentally stopping the experiment by pushing the big red emergency “Stop” button in the toolbar, which forcibly and immediately terminates the programme, not allowing the usual clean-up procedures (like saving data) to run. People will swear black and blue that they didn’t do it, but for an experiment that otherwise runs reliably, it is the most likely cause of this problem. Maybe the log file can give some clues as to whether that happened, but I’m not familiar enough with it to say.
Actually I just noticed this, which adds a new dimension to things. A forcible quit would only explain failure to save data at the end of the experiment, not loss of data that is saved progressively.