Javascript calculated field will not update, not calculation order error

I have created a PDF that uses a custom validation script to calculate the final score of an exam. I have gotten it to work a few times, then it stopped. There was no rhyme or reason why it stopped.

This is the script:

//variables
var bt = this.getField("BaseTotal").value;
var pm = this.getField("PenaltyMinutes").value;
var temp = 0;

//temp variable to store calc
temp = bt - pm;

//assign final value
event.value = temp;
 

I have verified that the calculation order is correct.