'Convert To Full Backup' greyed out

I've been cleaning up some of my backup vaults and it seems with some of them that have a mix of full, incremental and differential backups, I can't use the Convert To Full Backup option on some of them. It remains greyed out after clicking on any of the incremental archives. And yet other vaults clean up fine. What is causing this limitation? The only thing I've read is it can't be done when the backups are on tape or cd/dvd (for obvious reasons). But my backups are on a hot-plugged sata drive which has plenty of free space.

Any ideas?

EDIT: I should add this is with AB&R Workstation 11 personal vaults.