Problem adding meshes

I was trying to replicate dust 2 on ue4 and i got this situation.

I made almost all the map , walls and floor , but now every time i try to add a new mesh , it just doesnt spawn and if i try do copy with the alt shortcut the copy and the original bugs and they cut themselves and then the origial shows normaly again but the copy doesnt.

i will answer as fast has i can.
im new on this btw c: