I ran into this problem as well. Specifically an elevation marker was not visible in a plan view when I thought it should be. All the other elevation markers were visible. Phase & discipline were all the same.
While all the other things mentioned previously by others may be true, none of them were the answer for me (phase, etc.).
What I did find was that:
The plan has a View Range. The elevation view has Crop View settings. They have to be consistent with each other for the elevation mark to show in the plan view.
Specifically if the "Crop View" is checked in the elevation view and the top extent of the crop box is below the "Bottom" extent of the plan view view range the elevation mark will disappear in the plan view. If you move the top extent of the elevation view crop box back above the plan view "Bottom" setting it will reappear.
If the elevation mark is set this way so as to not appear and you uncheck the Crop View setting in the elevation view properties, the elevation marker will reappear.
This is repeatable. Tile the two views and you can disappear and reappear the elevation marker to your heart's content.
You sometimes have to let Revit catch up for a half second or so for the change to take effect. F5 seems to get it to regenerate views. So if Revit doesn't seem to be affected by an adjustment that should cause the elevation marker to appear or disappear in the plan view when it should, press F5 and it will.
The plan view View Range limits that affect the visibility of the elevation marker are "Bottom" and "Top"
The Elevation View settings that affect the elevation marker's visibility in the plan view are the draggable vertical limits of the crop view, and the Crop View check mark.
GChapp