I had the same problem with build 4.5.1. The “open level dialog window” did not see some of our maps. I fixed it by changing the default map of the project to one of the maps that was missing from the dialog window.
This option can be found by going to: Edit → project settings… \ beneath project there are some options for Maps & Modes. Set the default editor map to the map that is missing. Next restart the editor, and save the map. From now on it will be visible again from the “open level dialog window”.