I have the same happening sometimes, I don't know why. Sometimes it helps to just save the library before adding more objects. That is the only reason I sometimes have to use the import button, otherwise I only use drag&drop. Try to drop in the empty area below or beside the columns.

Maybe you mixed up mm and cm? I often create objects 10 or 100 times as big as they should be because the details are too small to edit in Sweet Home 3D. When you import it in the Library editor you can use the x10 or ÷10 to resize the object for usage in Sweet Home 3D without having to resize the actual object itself. You can see the values in the modify dialog when you click one of the buttons. You can also set a default elevation.

In the Library Editor you set the lightSourceMaterialName to the material name as it is in the obj/mtl files. Most existing light models use 'light' for the material name so you enter light as the value for the lightSourceMaterialName.
But sometimes you want multiple parts of the same object to act as a light, parts that have a different material name. You can multiple material names as the value for lightSourceMaterialName. Add multiple material names separated with a space. Be aware that multiple material names from the the same object will use a single power setting to control them all.