This is unfortunately a long-standing Nero bug. Nero 5 and early versions of Nero 6 created DVD video discs with the correct volume label length. Then Ahead broke it and now they pigheadedly insist 15 characters is correct, refusing to fix the problem. Why? Who knows, maybe they decided to adopt Microsoft's old habit of denying bugs. I'm really ticked off, I paid for this software and they could care less about sorting this out.
They're quite obviously wrong about the volume label length - I have piles of original DVDs with longer names and every other piece of burning software I have tried allows labels longer than 15 characters. Unless they can produce the document on which they based this, and explain why everyone else has longer labels, and why DVD players are quite happy with them, I am just going to assume they messed up and take my business elsewhere. In future my company will not be buying Ahead products.