when I got my A900, it also kept falling and slipping off it was loose also and fell off easily.
If I recall I did not have to bend it or anything but after a few weeks it just naturally fit and now it fits snugg and does not flop or slide off....
I guess the only differences in the way I used it are that my ears go inside the hole of the pad and before they were on the top of the ear pad...
Having my ears on the top was no good anyhow after a few hours use they got very hot and sweaty... use to get a ear rash also !
But I guess once I put my ears more inside the holes within the cushions it stoppped sliding off and can use it for many hours without sweat or getting too hot.
I also take off my cans every now and then to give some space/air to my ears and headphones.