StoreEasy 1440 deduplication problem

[M. N.]

I have a StoreEasy 1440 NAS used for backup. Deduplication is enabled.

I need to free space and I deleted more than 1 TB of data, but the free space increaded by less than 1 MegaByte (yes, MegaByte),

I realize that part of the files I deleted can be duplicated, but I can't believe that the free space increased by less that 1 MB.

I presume that something is wrong, in the storage or (probably) in the confnguration.

How can I troubleashoot and solve this problem?

Regards

M.N.