It sounds like you either don't have the usage rights for barcodes enabled for this form and you are using Reader to view it or you have exceeded the capacity of the barcode.
The interactive barcodes require extended capabilities that are not available by default in Adobe Reader. If you are using Designer and using Preview to test the form, make sure that you have Acrobat available on your development system. Also, if you have both Acrobat and Reader installed on your development system make sure that Acrobat is being used by Designer and not Reader. I have both installed and I usually start up Acrobat in the background to make sure Designer uses this instead of Reader.
If you are going to release this form to users that are going to have Reader then you will want to Reader Extend the form using LiveCycle. This will turn allow you to enable some Acrobat functions in the free Reader. More Info: http://www.adobe.com/products/livecycle/readerextensions/
The fields in your collection, are they prepopulated? How many fields are you including in your collection?
I would like to make a comment regarding reader extension. Please note that it is very difficult to purchase this product (please see previous discussion in this subforum). If anyone has figured out how to purchase this product, I would be very interested in hearing their recommendations. Is there anyone at Adobe or a specific reseller that can sell this product?
Jag_138's reply is correct. You need the barcode form right, #9, as explained in my blog at Datalogics. Note that Datalogics is now reselling and packaging LiveCycle Reader Extensions in simpler ways that can be embedded in processes or products without the impact of the full J2EE server installation of LiveCycle. You can read more on the Datalogics PDF Java Toolkit page.