Subject: Why do ice cubes 'stick' only in the bottommost of two stacked cube trays?

I have a standard refrigerator, with the freezer on the bottom.  There are two 
ice cube trays, one stacked above the other (little or no air gap between 
them), both on a wire rack positioned above the tub that holds the loose ice 
cubes.  The trays are standard, tough, somewhat slippery plastic.  The trays 
are filled from the same faucet at the same time.  Regardless of which tray is 
on top, the topmost tray will release its ice cubes with a simple twist and 
the ice cubes will fall cleanly into the tub.  The bottommost tray, twisted in 
the same way, will release only a few cubes unless first warmed slightly under 
running tap water.  Furthermore, the cubes from the bottommost tray will 
rarely come out cleanly; instead small chunks and shards of the cubes will 
remain stuck to the tray.  Except for the position of the trays, I cannot 
think of any other variables in which the two trays differ.  Why do the 
cubes “stick” in the bottom tray but not in the top one?

This question has nagged me for years, and nothing I learned in high school 
science (long ago) seems to answer it. 

Thank you!

Re: Why do ice cubes 'stick' only in the bottommost of two stacked cube trays?