Hi, sorry to cut in. Just like Ian states, in certain areas of the world, hard water seems to make SB (and JP) board discolour. I have this problem at my local spot.
I normally have to clean all my boards with roughly 6 month intervals. But it is very simple to do. I use a house hold cleaner called Cillit Bang Grim & Lime.
Use a damp towel, spay the Cillit bang on the board and wipe straight away with the towel no need to scrub as the board will turn white in a few seconds. Hose the board down, and it will look like new again. Im sure that they feel faster after cleaning
Word of warning, don't do this on your grass lawn (like I did first time) because cillit bang kills grass and you will end up with a stencil shape of your board on your lawn (I had 6 stencil board shapes on mine)