Missing faces/material on meshes when alt+dragging an object

I have a multiplayer map that I am designing for school. I have been working on this project for a couple of weeks, but now whenever I try copying a geometry block i.e. stairs, if it is overlapping an object, parts of the stairs and the other object will disappear. It still has a material and everything, but is transparent in certain spots.

How do I fix this? I can provide pics or whatever else.

is anyone here