There are several reasons this may be occurring. Check the following:
Did you roll the Grade Table over? After rolling the data base you need to Rollover Grades. If your 2015-2016 Grading - Maintenance Grades Table has no grade values then you need to Rollover Grades. Go to Administration > End of Year Procedures; Set Source Academic Year = Previous School Year; Destination Academic Year = New School Year; and Set Override if Exists = Yes.
Do you have Report Card Parameters set to “Convert Rankbook Alpha Grades for Number” = Yes and you don’t have alpha grades in your grades table? In this case the numeric Rankbook Averages will not know what alpha grade to put in the Grade Heading column. Either change the setting to No to report numeric grades or set up your alpha grades. Go to Grading System > Maintenance - Grading > Report Card Parameters to check your settings.
Do you have the Grade Entry Parameters “Do NOT copy Rankbook grade to grade column when entering grades” checked off? If this box is checked off then the Rankbook Average will not auto-populate the open Grade Heading column. Go to Grading System > Maintenance - Grading > Grade Entry Parameters to check your setting. Note, if you change anything on this screen and Submit, check the Report Card Parameters screen again to confirm settings are what you want.