I will throw in one additional, albeit not very likely, thing to consider.
If you were, by chance, attempting to print a non-standard character (or set of characters) into your barcode font, you might need to use another font set.
For example: There are 2 sets of Code 3 of 9 fonts - Standard and Extended.
The Standard set handles all 'normal' Alpha-Numeric characters
The Extended set handles all that the Standard set does, but also handles additional ASCII values such as ENTER (CRLF), TAB, Backspace, etc.
If you were going to attempt to print one of the 'additional' ASCII values using a Standard Code 3 of 9 font, it would not work.
Additionally there are certain barcodes (and there associated fonts) which will not support Alpha characters at all.
The UPC barcodes A & E
Interleaved 2 of 5 (I25)
etc.
If, you attempted to print Alpha characters with them, it also would not work.
Admittedly these are not likely to be the situation you are experiencing, but I thought that I'd throw it out there just in case.
Good Luck,
JRB-Bldr