I had the same problem on both my cars, one an Alfa 166 and the other a Volvo V70, which is that the fins in the airvents are not flat. On the Volvo they are curved, like the profile of an aeroplane wing. On the Alfa the vent don't really have fins and instead have two flaps but they are quite think and have a lip on the edge.
I found the only way I could get this to fit was by removing the rubber boot from the prongs. With the rubber fitted, the thickness of my vent fins/flaps meant I couldn't tighten the grip down enough, so the prongs would be in a V shape and it would just slip straight off the vent. With the rubber removed, it made the gap between the prongs wider so I could get enough purchase on to the vent, but then it has to be really tightened down as you've not longer got the rubber to prevent it slipping about. Getting it tight this way is difficult as the product is so small it's hard to twist hard by finger.
On a positive note, everything else about it is good. The metal piece has stuck fast to my Otterbox phone protector (as it's lightly textured I sanded it flat where I was going to stick it), the magnet is strong, and it's just about the most quick and convenient way you'll ever find to mount and unmount a phone in the car. It does what it says on the tin, in being able to hold a phone securely. Though that said, I suppose another down side to it's design is it leaves your device free to rotate (with the magnte being the pivot point) so you will find that your phone can move/swing/rotate etc with the movement of the car.
So my advice is, check your air vents before ordering. If they are thin and flat, this will probably mount up just fine and do a great job. Otherwise don;t waste your time, or maybe look at the other Tetrax version that sticks to your dashboard.