Access points with integrated omni-directional antennas (where the max
gain is sideways) are placed among or above metal air ducts, grids and
large lamp reflectors. Result: A lot of reflections and refraction cause
multiple copies of the signal to arrive at receivers, thereby dropping the
data rates. As the radio waves reflect in random directions, the signal
strength suffers at both ends.

The top AP should be at least 4 ft. higher (preferably more) that the AP
below. Do not place anything conductive near Access Points.