I'm a recently hired employee of a medium sized school district and have myriads of different types of HVAC equipment. I'm fairly new to Trane heat pumps, with their electronic defrost system, and am a little stumped. We have 2 schools with this brand of heat pump where one unit will not initiate defrost and the next one will (about 15 0f 30). Upon manual defrost, pressures on both high and low sides look comparable and in test mode will defrost. I've replaced the "silver bullet" thermistor sensor at the evaporator base, swapped control boards and cleaned seemingly pristine outdoor coils as a last ditch effort. What am I missing?

You are missing a qualified contractor to work on the equiptment. You have tried to replace all the parts and it still does not work. You might be missing the signal from inside unit to the outside unit telling it to defrost and operate the reversing valve.

I'm a recently hired employee of a medium sized school district and have myriads of different types of HVAC equipment. I'm fairly new to Trane heat pumps, with their electronic defrost system, and am a little stumped. We have 2 schools with this brand of heat pump where one unit will not initiate defrost and the next one will (about 15 0f 30). Upon manual defrost, pressures on both high and low sides look comparable and in test mode will defrost. I've replaced the "silver bullet" thermistor sensor at the evaporator base, swapped control boards and cleaned seemingly pristine outdoor coils as a last ditch effort. What am I missing?
You say you replaced the sensor at the evaporator coil? Did you mean condenser coil?(Defrost thermostat)If the unit has ice on it jump the defrost test. No defrost- jump out the defrost thermostat and try defrost test again. Did the condenser fan shut off and the reversing valve switch?No- Bad board, Yes- bad defrost sensor.

Trane has many types of defrost controls. Some of the control boards look at outside temperature and total compressor operating time before starting a defrost cycle. The sensor bulb you replaced may be for terminating the defrost cycle only and won't start the defrost cycle. I would need the Trane model number to answer your question acutely.