Implementation Restriction while querying Article Types

Always add language='en_US' AND publishstatus ='online' in the Query.

Sample Query:

String strSOQL = 'SELECT Id, ArticleNumber, Title, LastModifiedDate, CreatedBy.Name FROM Knowledge__kav WHERE language=\'en_US\' AND publishstatus = \'online\'';

Check the below link for more info

https://help.salesforce.com/apex/HTViewSolution?id=000113362&language=en_US

Cheers!!!