I had the same problem and cured it completely I thought it was the solenoids, but it is simply the small passages in the top of the dispensing tray that get clogged up with gunk. I removed the tray and took it apart carfully, there are some cleverly moulded flaps to open, a bit of a puzzle but worth the effort. I then cleaned out the channels with a feather.. yes a long pheasants tail feather, it works a treat.
Good luck it takes about 15 minutes.
Tim
June 2006
Turns out to be mould in the jets over the dispencer tray.
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