lhlo (demerits for picking a cutesy-poo "handle" for which I can't figure out whether the first letter is upper-case 'I' or lower-case 'l'),
First, read the boxed-in paragraph below the screenshot at the top of page 448 in the Retrospect Windows 9.5 User's Guide. If that paragraph describes what happens when your Member is full, un-check the checkbox and increase the "Use At Most" number of GB appropriately.
Second, if that doesn't work, do what it says on pages 449-451 of the UG under "Adding a Disk to a Backup Set". Add the new Member on the same disk as existing Member, and give it the appropriate "Use At Most" number of GB. This post describes that as the workaround for an earlier version of Retrospect Windows. So this is your "poor second choice", but next time you'll remember to set the "Use At Most" percentage appropriately when adding all new Members.