Laura,
I'm using iBird Plus so I'm not 100% positive what's going on in your case, but I would bet that you use more than one computer or more than one computer login account. If this is the case, the most likely scenario is that your iTunes App Store authorizations are the problem. Make sure that under the Store menu in iTunes that you have the correct user credentials (by correct, I mean the same credentials under which you originally purchased iBird). You may have to deauthorize and then reauthorize the account (which doesn't hurt in any case). I've found that only syncing the iPod/iPhone with a single machine helps keep my sanity.
If this is not what's happening, I would recommend calling Apple for support (unless someone on this forum has a better idea).
Keep us posted as to what you find out,
Robert