I have never worked with that plugin, but if you want to change the sorting based on an existing meta value ( city, id, distance, street etc ), then this article explains how to do that.
I have tried using the function from this article but the function only works for meta fields begining with “wpsl_“.
This can be found in the class-metaboxes.php (wp-store-locator/admin/class-metaboxes.php) file, on line #604
If I can the meta key to a generic key then I get an “No results found” message with the map. I’ve also flushed the transient cache on the WPSL settings page -> Tools section.