I'm trying to hang a picture on a sheet rocked wall and I keep hitting something behind the sheet rock that "rejects" my nail. It is not a wooden stud. My nail kind of "bounces" off this stuff and cannot be pounded into this stuff no matter how hard I pound. I've had this same trouble in an apartment wall and was able to get a screw through the "stuff". I'm just wondering what is this "stuff" and why can't I penatrate it with a nail...