If it is the 'coffin' type they have a rather nasty habit of going a bit gungy where the bare alloy inside reacts with the brake/clutch fluid and the moisture which they absorb and will easily block the hole into the piston itself which may well be the cause of it not working properly.
If it is seized up then obviously you have other issues in which case I would get the whole assembly off the bike and see if it can be freed off.
If it has the PS13 master cylinder then you can buy a kit here:
https://www.ducatipaddy.com/page21.html