Reasons why a rear defrost does not work properly

Hi, I recently purchased a new 2014 Honda Odyssey Ex-L.  The rear defrost has never
worked properly. Initially, it never warmed up or produced any heat at all, despite the light
being on. When I first brought it into the dealer, they reported it worked for them. I asked
them to show me and when we went to the car, the middle section of the rear window was
hot and the sides of the rear window were cold.  They then "worked" on it and showed me
that the entire back was heated. They claimed it worked and there were "no issues".  I
took it home and tried the defroster repeatedly over the next few weeks - there was no
heat produced at all the vast majority of times and on one occasion only, there was heat
uniformly across the back window.  The rear defrost light has always lit up and I have
always tested it after 10-20 minutes of being "on".  There are no other issues with
anything else in the car. I brought it back to the dealer for service and they again say that
it works for them and they are "at a loss" to explain my experience or fix anything (since
they cannot identify what to fix).   Any ideas on what could be wrong and what they are
missing?  And yes, as I explained to them, I do know which button is the "rear defrost"…