Items in forge not spawning in custom game

I’ve saved someone’s map of rust into my file share but when I try to edit it in forge, there are a few items that can’t be selected. The issue is that when I load the map in custom game, the items that can’t be selected in forge disappear from the custom game so there are lots of holes in the map.
Sorry if this question’s already been answered but i’m still learning how to use forge and this is really bothering me. Thanks!