Koolstop cartridge pads not fitting

Last night it took me nearly an hour to replace my V-brake cartridges.
The koolstop pads I'd bought of the internet simply would not slide
into the holder - to the extent that I (a) had to pare the backs of
them down a bit and (b) remove the holders from the bike and use some
molegrips to ratchet them into place.

Am I just unfortunate in this respect? Did I buy the wrong pads?
Anyone else had problems with them? These were the green ones for
ceramic rims (something I'd never have bought, but it's a second hand
bike).