Often when you design an RDLC report, you get asked to add columns to it after some time. You may need to change the database query in order to get the data for the required columns.
In order for RDLC DataSets to contain the new columns, you need to go to the DataSet in visual studio first (xsd files, locate the xsd files e.g. in the root of the Visual Studio Solution), in which you can add the column. Next you need to go back to the RDLC report, and Refresh the DataSet of the report. Now you can add the columns to your report.