You may be loosing results due to properties that are do not apply to all computers. If one property only applies to Linux endpoints that would cause all Windows endpoints to drop off the results. Likewise if you have a property that only applies to Windows endpoints, all the Linus ones will drop off the results leaving you with an empty result.
I would look at @brolly33 blog that has been referenced in your other forum post and test using either the presentation debugger in the console or the stand alone session relevance tester so you can develop the correct session relevance expression.