Product review (2 stars): They need some help...
Well, echoing what others have said here, these just don't have the magnetic power to hold onto a vent.  Searching similar products I don't think any of them really do.  However, I expected this going in so I also purchased some little powerful magnets to help hold them on.  With the magnets I have these up on ceiling vents that have a pretty substantial air force coming out of them and they stay on and block the air well.  It helps to clean your vent with some rubbing alcohol prior to installing these to remove any dust and get cleaner contact.  The magnets I bought with these here on Amazon are:  10 Neodymium Magnets 1/2 x 1/8 inch Disc N48, UPC 033383268422&nbsp;<a data-hook="product-link-linked" class="a-link-normal" href="/10-Neodymium-Magnets-1-2-x-1-8-inch-Disc-N48/dp/B001KV38ES/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">10 Neodymium Magnets 1/2 x 1/8 inch Disc N48</a>.  Hope that helps others, good luck!