All 51 responses are marked as complete, but what I get is only 27 responses in the CSV/XLS.
Also when I try to export the remaining responses, the CSV/XLS is empty.
How are you exporting to CSV?
Are you exporting from the Individual Response page directly or from Results > Exports?
I recommend trying to export from Results > Exports and making sure there are no filters in place that would prevent certain responses from being exported:
I hope this points you in the right direction!