Migrate that Account's (user's) OneDrive OR check the Chat Migration configuration option: 'Skip OneDrive Status Check'.
If the 'Skip OneDrive Status Check'. is not checked, any users who have been in chats need to have their OneDrive migrated, if you want those chats to be migrated. Let's say user A and user B have chats with each other together. And user A has their OneDrive migrated but not user B. What will happen is the migration of that chat will be skipped. For a chat to migrate, all of the participants in that chat need to have their Account matched and their OneDrive migrated. Otherwise that whole chat's migration will be skipped.
It can take awhile for the On Demand Migration (ODM) tool to recognize that. If you immediately run the Chats discovery, it may still report that the OneDrive has not yet been migrated. Microsoft just needs time to show us the information requested. When ODM migrates it migrates into a blob and then Microsoft places it into the correct place once it has a process available.
