I wondered why my merlin (on low pressure) had gradually drifted from about 40 litres an hour of pure down to about 15. It had been a fairly gradual decline over a year but had recently accelerated.
Well I finally let the moths fly out of my wallet and changed the membranes. They were a very dirty dark brown - oops !! I put new ones in and POW - back up to where it was a couple of years back.
Serves me right for being a tightwad