Storyspace Guard Fields

I’ve double checked that I’m creating a text link from Third Note to Fourth Note, and, unless I am being more than usually obtuse, that is what I am doing. When I complete the creation of the link it appears in Map view, with a “no entry” sign attached to it, which seems reasonable. However when I start to coerce Third Note to be visited the requisite number of times the link disappears in Map view, and the Guard Field does not operate. Attached is a screen shot of what I see immediately after creating the link, showing the link with the “no entry” sign. However that disappears once I click on Third Note the first time.
I’m resetting $Visits in Third Note to 0 by having it as a Key Attribute, double clicking on it in the field at the top of the note and setting it to 0. Could that have something to do with it?