I too had this problem which is answered at https://support.mozilla.org/en-US/questions/1372085. Although it provides a work around to the problem it does not '''solve''' the problem. The SUBJECT column is an essential part of the index and if it is not showing there should be a way to select it as one of the columns. There is even the scenario where the problem I had applied to all the folders and there was no folder to copy/paste to those where the subject was not showing!