No multiple recovery points being kept

I have Norton Ghost 15.   For a long time I have been backing up to a 500GB external drive and had set a limit of 3 recovery point sets.   This drive eventually became too small so a couple of months ago I attached a 2TB drive and created a new definition to start backing up to this drive instead.   In the Define Backup Wizard I have set the  "Limit the number of recovery point sets saved for this backup to maximum 3.    

I create a new recovery set each month and have just run the first backup of April.    However when I looked at the Manage Backup Destination I only have the latest set on the 2TB drive.   I was expecting to see March also.   The checkbox entitled "Limit file versions for file and folder backups" is GREYED OUT.    (on the original 500gb drive,  this is not greyed out and I was able to have multiple recovery sets.)

I can find no reason why this is happening.    I altered the limit the number of sets to 4 to see if I could force an update but this has not made any difference.   Can anybody please advise as to how I can get more than one recovery set onto the 2TB drive?    Thanks