I've come across that problem, too. If one file of a collection (nzb) is missing completely, the whole collection won't be unrared, even if there are enough par2-files to cover the missing data. But, if you download further par2-files manually, alt.binz starts to repair the collection und unrars it.
However, I have to say, that's not a very frequent problem, I would consider it a minor bug :wink: