If there is an ERROR or the length of the number if too long, you will get the "?"
Can you test in the Data Viewer to see what the error might be?
Or can you resize the field (wider) to see if that's the problem?
IF you divide by "0" that is an error.
You can test for possible problems and return 0, for example:
X = If ( B = 0 ; 0 ; A/B // test for 0 )
Division by zero is mathematically not defined.
So you should trap the case where the divisor can be 0.