NX Cannot constrain some components of an assembly (Restrict to Interface)

Assembly constraints cannot be selected even though all components are loaded

In this example it can be seen that one of the loaded components (gray) is not selectable. Assembly constraints can only be selected on one component (blue) .

If "Restrict to Interface" is enabled for a component in the Product Interface, it can no longer become constrained.

In this example it can be seen that one of the loaded components (gray) is not selectable. Assembly constraints can only be selected on one component (blue) .

The gray part (which is a Part Family Member) has a Product Interface on it. When trying to select the gray component, this is not highlighted for selection.

To fix that make, the gray component to the work part and then go to Menu > Tools > Product Interface and notice the  “Part Referencing Rules” for Assembly Constraints is set to “Restrict to Interface”.