The editor filter operator acts as a logical 'AND' statement. So, for example when you search for strings with the following criteria:
translated:no (it returns all the untranslated strings)
proofread:no (it returns all the translated strings that haven't been proofread yet)
the system tries to find the intersection of those two sets and as a result, no strings are returned.
But if you apply the following filters, for example:
issue:open (it returns all the strings with open issues)
the system will return all the untranslated strings with at least one open issue.