Threaded hollow wall anchors spin out partway in
i don't understand why you just didn't anchor to the studs, but you must have had some reason.  

sometimes the metal anchors are a little too hard for the screws, so the torque that is exerted on the screw translates into the anchor, and the anchor spins in the drywall, thus defeating the purpose of using the anchor in the first place.  you can't use those anchors in those holes, however, you can get yourself some toggle bolts and put them right into those holes.  they are a stronger anchor anyhow.  have a look here

https://www.youtube.com/watch?v=_IuNkiiqhO4

just use plain old toggle bolts, not the plastic ones or the newfangles ones or the usb connected ones.  just plain old togglebolts.