Sub-Zero Under Counter Ice Maker not making Ice

Question

I have a 9 year old Sub-Zero under counter ice maker that is having problems making ice. I have cleaned the system thoroughly, including the freezing tray. When I turn on the ice maker the pump starts to pump water up to the freezing tray and the water runs down over it. After a short time a sheet of ice forms on the freezing tray but it never releases and slides down to the cutting tray. I have let it run for several hours and the ice never releases and gets cut. Any suggestions would be welcome since I feel like I have tried everything except replacing the freezing tray.
Thanks in advance.