I had the same issue and it was not immediately obvious what I was doing wrong.
When you create or edit a database, make sure you have actually selected a user from the list of "Read/Write" and/or "Read Only" users. The UI will show you a list of users, that is not the list of users with access. That is the list of user you can select from to grant access.
The main database page will show which users have access to each database directly to the right of the database name field. If there are no user's listed in those fields, you'll have to click the edit (pencil) icon and select a user from the list.