Some event organisers choose to suspend all barcodes until a date closer to the event.
This is usually the case for high profile events, in order to combat tickets being resold at extortionate prices, touting and unauthorised duplication of tickets.
For example:
If you're passing on the tickets or bought them as a gift then you can update the name\and or transfer the ownership and the recipient will have access to the barcodes once the tickets are available to download. To find out how to do this click here.