Plots with subplots aren't expandable to show subplots until the main plot entry has been clicked on. This only appears to be an issue when first loading the article tree (maybe).
Have a plot with one or more subplots.
Open the Articles & World Manager tab.
Hover the main plot entry - no arrow appears to expand the node and display the subplot articles.
Select the main plot entry so that the editor loads.
The option to display subplots within the tree should now appear.