Are you getting some error message, such as “unable to perform query” ?
If not, what happens if you navigate to one of the items in that tab and manually execute the query (enter query -> execute query) ? does it work ?
If yes, a relation between the master block and this block is missing.
If not, check the ‘default_where’ property of the block. It could be causing no records to be retrieved. If the data source type of the block is other than “table”, you might want to check also the stored procedure or query that gets the data from the database.
If this doesn’t help, please provide more information and I (and others) will try to assist you.