Some Bar codes require a Start and stop code at the beginning and end of the string. It is not enough to just format the string with the barcode font. you need to use a formula to add the required start and stop codes ... and sometimes a "check Digit" as well. They are all a little different and have their own peculiarities Even if the code looks correct you should verify that it is working correctly with a bar code scanner to confirm that it actually works.
Ted