What may be happening is that your form has not been properly "prepared" for publication to the free Reader. You have to first run your form through Reader Extensions to enable the Barcode technology.
no this was not a problem. you can publish a form without preparing it through "reader extensions". you will be not able to decode it, but the barcode will work.
the problem was: i tried it with adobe reader 7.0. i designed it with adobe designer 7.0.8. so it works with reader 7.0.8 and 8.0
Yes, you are right, if you do not "ARES" a PDF the barcode should still work but what you may see is that the Barcode gets encrypted. When the barcode is encrypted some of the space of the barcode is used up by the encryption process. So, this may cause a barcode that is filled to capacity perfectly fine in Acrobat to gray-out in Reader.