Any suggestion as to what is going on here and how I can possilby fix
it?

Living in an old house requires learning the art of shimming. I'd say
there is space between the floor and the floor joist. If you have a
basement, no problem, if you have a crawl space and are slim and
athletic, no problem. Get a package of wood shims and a hammer and
flashlight, get an assistant to tap on the spot, fill the gap. If,
however you are on slab, disregard this post.