i've seen a couple of odd problems with induced draft blowers. i learned the hard way that a speck of carbon can get trapped in the nipple the hose connects to, blocking airflow. after replacing the pressure switch, i realized i jumped the gun. now i carry a paper clip in my tools when i service furnaces. i've seen this problem 4 times.
another time, screws holding the housing to the furnace had backed off due to vibration and the air bypassed the gasket for the housing. there was not enough negative pressure to close the pressure switch.