If a gradebook item has an invalid calculation then gradebook exports simply stop with no information if the course item needs an update.

We've seen issues where the course item calculation has become: =sum(##gi27096##.1,##gi27093##.4,[[S2Ind]].35,max([[S2Quiz]].3,[[S2xtra]]*.15))

Which we're guessing is caused by the deletion or replacement of activities that were being used in the calculation.

The issue isn't that it doesn't return any grades (after it would be bad to return "bad" grades) but that it doesn't indicate that this is what may have happened.