Inexperienced crew or they didn't read the instructions.
We had a new furnace and AC unit installed years ago and when it came time for heating it didn't work. I found a wire that apparently fell loose from a circuit board hanging in plain view, but I called the installing company anyway. The original Tech who installed the unit came and with minutes after he remove the funace's service cover he stated that the the power switch next to the furnace was in the off position. No mention of the loose wire that I found now reattached after he left.
Go figure.