I am making the assumption that the data table you are having an issue with, is one that you created from the Summary Platform, or maybe from the Subset Platform. In both cases, the Locking came from having the "Link to Original Data Table" selected. You can remove the lock by either deleting the table, and rerunning the Summary or Subset with the "Link to Original Data Table" unchecked, or Save and close the data table, then reopen it, and go to the Column Info for the column in question, and unselecting the "Lock" check box.
Jim