Mullions not showing up in elevation view

[roscopeco2000]

is there any reason why a mullion shows up in a 3d view but not in elevation? see below examples...

[ToanDN]

One reason is the glass is in front of the mullion.  2D elevations don't have transparency through fenestration like 3D elevations. 

[Sahay_R]

Have you looked at the Visibility Overrides for the Mullions?

If that doesn't work - can you share the family?

[roscopeco2000]

its the standard one that comes with revit details below i couldn't see visibility settings in the properties so i went into the family an elevation box was ticked. I tried setting round it by making each pane of glass fit inside the each one of the 4 frame but when i went back to project the windows panes were misaligned.

[ToanDN]

You have to constrain the glass panes to the mullions, or to the reference planes define the mullions, so that they stretch properly when the window size changes.

But, i don't believe that is the source of your problem of mullions not showing. The problem is the glass PLANE is not constrained properly, so when the host wall thickness changed, the glass plane shifted to one side to offset to the mullion plane, causing it to obscure the mullions. You can check by create an elevation from the other side of the window, you will see the mullions.
Or create a section thru the window, you will see the offset between the mullion plane and the glass plane.

[Anonymous]

I think it might be because of the type you chose. One half has the mullions in the interior side and the other half has them in the exterior. So as @ToanDN mentioned you can not see them through the glass in the elevation.