Hmm, it appears I am having the same problem describe by bitter but alas no solution was ever posted. It also appears that bitter's explanation and problem was never fully understood.
I have the same exact problem. I have bifold closet doors in several rooms that had old ugly brass knobs installed. We are replacing them and have had minimal luck.
The issue is NOT too long of a screw, which can be easily determined by a) using a shorter screw or b) some sort of spacer like bitter used. Instead of a nut I had a washer and to space it even more I stacked about 6 washers.
The problem is that no matter how tight I screw the screw into the knob, with little force the knob just spins freely in the hole. There is no back and forth motion (indicating a too long screw) it just simply spins. I can see the screw head spinning on the washer.
It would seem the intent of the design (for newly installed bifold closet doors) is that you drill a hole about the size of the screw being used and simply screw it down. The clamping action between the screw head on the back of the door and the knob on the front keeps it fixed. Kitchen cabinet doors work this way and I have other bifold doors that don't seem to have this problem but there are no differences in design.
I could use other tricks like getting everything in place, filling it with glue and then tightening down the knob and then hope it holds but the door is hollow so not sure how this would work.
Very confusing, maybe the hole is simply too big, but again it seem like the clamping action should do the trick. Any further suggestions would be greatly appreciated. I have a bunch of closet doors with no handles and my wife is mad!