Try something newer then. If this is on the edge of retention, only getting some of the files is pretty common. If you watch the logging messages, you'll see whether the chunks all downloaded or not. 430 errors mean the server is saying "I don't have it". As I said, Newsbin will automatically assemble what it has of the file even if it's not complete in hopes that the pars can fix it.
If the files are old and the server is saying "I don't have some of the chunks". Assembly and hoping repair works is about all you can expect. That's why a fill server that fills in the missing chunks is a decent investment.
I do have free usenet account with my ISP, I suppose I could try using that as a slave for now and see what happens.
Might be good, might be worse than your current server. I'd set it to "Fill" so it only gets used for missing chunks on the primary server.