How can I fix this loose chandelier mount?
With another floor above try removing the plate on the face of the box and see if there are some screws that are loose you can tighten, if no screws , push the box up is it hitting something? That will be a cross brace to screw into. The box is usually less than 2-1/2 deep and with a floor above there should be 6-12 “ clear above the box other than the brace, I have had nails come loose and used screws to reattach the boxes in many cases over the years. The box may also be attached on the side in that case shooting a screw or 2 into the joist would work.