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.