If you are using Relevanssi, it is messing with the native search (don’t know exactly at which point), thus, the standard query doesn’t work.
I have tested a solution, according to what I read in the plugin’s page, and it seems to do the trick. Just run the relevanssi function to append the results to the query, in this way:
Note that the parameter is the query object you created before:
$query1 = new WP_Query( array( 'post_type' => 'sdm_downloads', 'posts_per_page' => 3) );