Sometimes there are posts with the pars named in a way that NBP doesn't recognize 'em as something to put into the scratch list.
A simple solution (using find)
foo(?!.*\.vol)
finds all of foo without the string vol (assuming par volume posts say "vol" somewhere in them)
select all, then [enter] or [ctl][e] - to put into download list
then (again using find)
foo.*\.vol
select all, then [ctl][ins] - to put into scratch list
More of an issue for multi-volume stuff, of course...