How do you open the section?
If you are clicking on the view, what is the layer combination associated with the view?
Use Check Duplicates in the plan with all the layers on and unlocked to see if there are identical objects occupying the same space. If there are, as Peter said, they will not show.
Also, is your section line maybe of zero depth? In that case, only the cut objects will show in the section.
Djordje
ArchiCAD since 4.55 ... 1995
HP Omen