"sort" option in query does not include results that have no value in sorted field
From semantic-mediawiki.org
Fragment of a discussion from Semantic MediaWiki Community Portal
A simple workaround to this is to define, in the template, some default values for the properties of interest if they are not already set. ie:
{{#if: {{{ArgIwant|}}}||[[PropIneed::SomeDefaultValue]]}}
It's more elegant to do it where it's actually printing, but this has overcome the whole 'sort' issue any time I've needed it. Though, it assumes all the articles call some templates to which you could do this.