How to modify VSS retention ?

Hi,

I have a Windows 2012 Server with DFSR role and a lot of data on many partitions.

All VSS are configured on a dedicated partition Z: for all other partitions. For one partition (S: 2,5To) i configure 200Go for VSS and i always see less than 5Go used. When i check available shadows copies there are only 2 versions.

Why windows doesn't use all available disk space to keep more versions ?

Thanks.

ps : sorry for my spelling mistakes.