backup keep only six copies

Hi,

 I am backing up virtual machines by Windows Server Backup and everything work with exception of one server - RDS. The machine is backed up but Windows server backup keeps only last six copies altough there is 272GB of 459GB of free space. This is not he case for other servers(file, print, database, dc,...).

 Is there a way how to force WSB to use all the space?

Thank you.

Pete

sfs