Please take a look at the attached file called Test Project. This is a dumbed down version of a problem I came across earlier today with a more complex window family.
I've got a wall with two windows which look identical in elevation. I've cut two plans, one at 4' and one at 6'. In one of the windows i can't get the vertical mullion to show up in plan regardless of where i cut and in the other i can't get the mullion to go away regardless of where i cut. the only difference between the two window familes is that in one the vertical mullion is joined to the frame and in the other it isn't. they show up fine in all other views.
It's 3.30 am here so maybe i'm just missing some fundamental issue but this is the first time i've had window families behave this way.
I'm on Revit Architecture 2012.
Edit:
I've attached the original window family that got all this started (Test Project2.) Most of the mullion profiles in this window are made up of sweeps and i can't get them to cut correctly in plan. Much like my question above the family display's correctly in all other views and in the family editor just not once loaded into a project.