Slice : returning rows with a specific value

[d0ugJ]

Hello, seeking help please. 

I have a google doc with each row containing a lesson for that period of the school day. I am trying to use the slice tool to only show specific classes in a view. I have typed the column title and class code into the expression builder eg: [class]=11Db2. For some classes it works and only shows rows with that class code, for others all rows are shown and it does not work. Im confused. Any help would be much appreciated!

Thanks,

Doug