Have seen 2 people report that they are unable to disable the common word threshold by setting it to 0. I have done the following to correct the issue, besides rebuilding the search index:

In includes/search/fulltext_native.php find:

Find:

				if ($row['word_common'])

Replace with:

				if ($row['word_common'] && $config['fulltext_native_common_thres'] > 0)

Another way to resolve would be to change the text for the feature and give the option to rebuild the search index but this is much simpler.