The Solution: Top up the softener tray with extra water so that it is up to the 'max level' mark.
As mentioned by Dan, the softener tray works by a siphon action.
The reason the tray does not empty is that not enough water is added by the machine during the softening cycle, so that the machine does not completely fill the softening tray; only when the softening tray is filled beyond the 'max' mark does the siphon action works. A partially filled tray will never empty - it's impossible due to the siphon.
The reason that not enough water is added to the tray during the softening action is that the water inlet of the machine is partial blocked (you can try to fix that) but it is simpler to just added more water to the conditioner tray.
Thus, the easiest way to ensure that the softener tray empties every time (after adding the softener to the softener tray of course) is to top up the softener tray with extra water, top it up to the 'max level' line (keep a bottle of water by the machine). That way, even if only a little water is added to the softener tray during the softening cycle by the machine it is enough to trigger the siphon action and the tray will empty!
Works for me every time, and no need to remove any parts - I used to take the tray out, clean it, scrub the outlet holes that fill the tray etc, but they used to gum up again quite quickly!
Hope that helps
MEB
February 2010