It happened that the usual compact disk from Hyper-V Console doesn’t seem to be effective on one of mine .vhdx file.
Looking around on the web, I discovered that it is a quite common problem.
The solution is to open a Powershell window with administrative access rights, and use the following command:
optimize-vhd <offline Hyper-V disk file> -mode full
For further details about this command, jump here on MS site.