Importing object to Sims 4 studio

Hi. You started a package from an item made of two or more meshgroups, whereas yours has only one. When you import it, only one, the shadow plane, is being replaced.
Clone a new maxis item, export the blend, open (always start like this). Check how many meshgroups there is, what cuts they have. Select an object and check how it is adapted ( bones, uv maps, vertex paint ( if there is one). Your custom object must replace the maxis one, so delete the maxis one, append ( or create) yours, give it the same cut, make sure it is assigned to bone(s) and uv maps are named the same. After that select the shadow plane and adjust it to fit your object. Save, import