Hi,
I think check if the other form developed by you and this one when printed from same printer and read with same reader how it behaves. I agree that readers are also specific to barcodes used. Also check if the window or place where the barcode output is displayed is having the same character paragraph formats and same fonts and they are not overridden by any other format like paragraph formats defines for windows or box etc.
Better to try and put it in a separate transparent window and check.
Cheers,
Arindam