I think it has something to do with the parentheses. I changed it to latex form and it worked:
cellContent(3,3): when table6.cellContent(3,1)="2\\left(3a-b\\right)"
You might want to put a warning in the note to make sure the students don’t put any extra spaces in the expressions. If they do, all these will get marked wrong. Checking latex can be a bit tricky. There are other methods, but you might not have time to change the code (depending on when you need to roll this out).