On the other hand, setting the LTIMassUpdateInterval should cause the grades to be updated after the interval passes, though there have been numerous reports of this not working properly. Check your apache error logs to see if there are any errors showing up with the mass update (I believe that's where they get logged. Someone can correct me if that is not the case).
If the mass update is failing, then the only other way for the grades to get updated in Canvas would be to reopen the assignment, and have everyone submit another answer.