I need a hand(y-woman)

I recently moved and although I am mostly settled in, I still have my floating shelves to put up. This time I said to myself, I will not do this alone cause I always fuck it up. I didn't have a stud finder and I began to grow impatient waiting for one of my handier friends to come over and help. So yesterday I gave it a go. I knocked on the wall. rap rap rap rap. I moved a little to the left -  rap rap rap and then a little more left - rap rap rap rap - yes, that sounds less hollow...ok, I found the stud! I start screwing in the screw (don't go there) and oh no, it slips right in super easy (again, get out of that gutter) and slips out just as easily (hey). I don't even need my screwdriver to get it in and out. (raise brows) So, I guess I didn't find the stud after all, so the great shelf mounting attempt (mhmmm) ended with a lot of empty holes (ACK) in the wall and me completely unsatisfied.