wp-dtree Links Disappear

Forum:
We have wp-dtree installed for the left navigation area of our web site:
https://studenthousing.stanford.edu.
Sometimes, when you click to go to another page, when that page is displayed, the whole left navigation area disappears.
I am not yet able to identify the pattern for this, but it seems to have to do with jumping from one node’s links to another’s, perhaps on the same level?
I would appreciate anyone’s help on this matter.
Thank you,
Red thru Violet