Some times workflow stops due to an error. It says 'Error updating a list item' and the outcome says 'Document checked out'.
Document gets locked when there are multiple fields set from the workflow simultaneously.
Set the 'Checkout' action before the other action items in the workflow. After the updates, set the check in action. By wrapping the workflow actions with the 'check-out/check-in' actions document will be available for all the updates.