Curtain WaIl Segments - Alternate Workflow

When you want to add or remove mullions along a gridline or grid line segment.

You typically would use the Mullion tool under the Architecture Tab but sometimes it does not let you place a mullion.

Here's the part that (to me) wasn't obvious.

When that tool isn't working for you.
Instead, with no command active, select the gridline that you want to add the mullion to.
Then, click the Add/Remove Segments button that is available when a gridline is selected.

You should be able to click and add (or remove if desired) mullions to the desired segment.