Why are there data sets with my export with quota=0?
Has the "Recalculate quota" been used while the survey was live?
When doing so, all quotas are being reset to 0 and gone over anew. In the course of this it might happen that completed participations are set to quota=0 and thus quoted out.
This is also stated in the corresponding warning-message which is show when clicking the button:
"Warning: Recalculating the quota may take some time. If your project is active while this happens, participants may enter the project during the recalculation and fill the existing quota. Thus, participants who have already filled the questionnaire may be quoted out retroactively. Do you still want to recalculate the quota?"