Is it a new release? Sometimes MakeMKV doesn't have the keys updated on the server for super duper new releases.
When that happens I usually have to go to FindVUK here. Then I place the keydb.cfg file in the directory MakeMKV is installed in. It should be smart enough to detect the keys locally if put in the right location and use those. The updated keys allow it to decrypt these newer disks.

AnyDVD is down due to unknown reasons currently. If your want to decrypt Blu-rays, DVDFab has a PASSKEY for Blu-ray may help you.