You have subscribed to receive certain messages from MessageMaster, but you are not receiving any, or you're not receiving all of the messages you expected to receive.
Check the Filters that make up the Subscription in question. Check for inadvertent errors, or for Exclusions rather Inclusions, etc.
Try executing a search that matches your filter, and see if you get the expected result. At the same time, you can use this technique to verify that there are in fact messages in the database that satisfy your criteria.
Finally, if using search strings, be aware of how they work slightly differently when used in filters vs. when used on the search page. Also be aware that search strings rely on a healthy full-text catalog and Microsoft Search service on your SQL server. Only the Message and Title are searchable fields.