I cannot select "Average" for some data columns

Hello,

I have my report linked to an excel with a list of student names followed by columns for scores on 5 different assignments administered 6 times throughout the year (30 columns of data). I want to be able to show the average across students for each individual column. The issue I am running into is that only the first data column of each assignment allows me to select Average. The following 5 data columns only give me options for "first", "last", "count (distinct)", and "count." I have noticed that the columns that allow me to use the average function have the sum all symbol next to them. Am I able to fix it and average each column?

I have included a picture of my options when working with the "Mechanics 1" data column which gives me more options, as well as the "Mechanics 2" data column which does not give me the option to average. 

Thanks in advance!