Tips for cleaning Zyx airy 3

I recently took my Zyx airy 3 to a friend, who looked at it under a microscope. To my horror, the diamond was covered in a ball of dirt and could not even be seen. This is despite cleaning the cartridge with zerodust and a brush every day. All my records have been cleaned by an Okki Nokki and I take great care to remove any dust before playing.
The stylus has now been cleaned and is back to normal and I have ordered a stiffer brush to clean it with.
Has anybody else had problems keeping this cartridge clean?.