Welcome to the forum, @Shijakari
As @Unatan2 and @Gaddiel mention, the decoration is usually placed when the rest of the building is finished.
Hearthlings might get blocked by some other object under the lamp and in that case they won’t be able to place the ladder.
Until you don’t have the lamps in your inventory (either crafted or bought) they won’t appear as a ghost to be placed in the building.
In any case, could you upload a savefile where you are running into this problem so we can take a look at it?
To upload a save, locate the folder in
C:\Program Files (x86)\Steam\steamapps\common\Stonehearth\saved_games (assuming a default Steam install), zip the individual save’s folder (not the entire
saved_games folder), and upload it. If the
*.zip is less than 10 MB, feel free to upload it directly to the Discourse. If larger, please upload it to a cloud storage site like Dropbox, Google Drive, File Dropper, etc. and post the sharing link here.