I wonder if the hot water has something to do with it too? I use only warm, and mostly cold to wash my nappies and covers, and I've had my Bummi's covers for a few years and they are still fab.
I admit that i have about 10 covers to rotate though

I think Bummi's are one of the best ones on the market - but I bought a wool cover from Darlings Down Under last year - and it is my new favourite night cover to put over a pocket for 'just in case' leaks.........