I have found the query service to be consistently (*almost* always, over the past several weeks at least) missing some items - an example is Q30252826:
SELECT ?item WHERE { ?item rdfs:label "Technology Centre"@en . } SELECT ?item WHERE { ?item wdt:P856 <http://www.tc.cz/en> . } SELECT ?item WHERE { ?item wdt:P2427 'grid.17033.36' .}
these occasionally return a correct response with this one item, but usually return nothing - either in the WDQS GUI or via JSON download. There seem to be a handful of missing items - Q30252828 is another. However, Q30252827 is fine:
SELECT ?item WHERE { ?item rdfs:label "Mediterranean Center for Environmental Studies"@en . }
returns Q30252827 consistently.
It's a little disturbing to have the query service completely missing some of what's in wikidata.