Hello everyone. If you are stuck with one last soul cluster remaining, there is a tricky little thing you can do. In the map it shows that there are 1067 collectibles. If you point the cursor on each of the districts/regions of the map, it will show that certain district/region collectibles.
For example, my last soul cluster was in the Barrens (Blackbeard area)
Why? Because I pointed my cursor on Barrens and it showed that I have 97/98 collectibles there. So I searched the Barrens and found it in a couple of minutes. Do the same thing for all the regions until you know in which one you are missing one last collectible. This wont solve the problem, but it will give you an area of where to search :)
Hope this helps a little.