Refrigerator ice question?

I have 4 refrigerators and 2 ice machines that all have the same problem. They make ice but the ice at the bottom of the tray freezes into a solid lump and is not usable without removing the tray and breaking the ice with a hammer. I first thought that frequent opening of the door affected the ice, but some are rarely opened. I do live in a warm climate but the top ice is being made and not thawing. All friges & machines are clean and well ventilated. The temp is right on all. Any ideas????