Put in a ticket to Apple Enterprise with those serial numbers so they are aware of it. We've been working with them on a few devices like that. What happened with some of them is that when we purchased it, the purchase became archived in Apple's systems so they have to unarchive that purchase on your Sold-To account so it can be active.
Going from the various questioned asked oft he Apple guy at a recent DEP presentation I attended...
As @don.cochran said, you can add them via order number if your retailer can make these available, serial numbers should also work.
If my understanding of the process is correct adding any device is dependent on the retailer your bought it from having it correctly recorded against your organisation, so if you have devices that won't show up you may need to confirm with that this is the case.