Sometimes underextrusion maybe the result of a fan shroud that might have been accidentally bent.
The Image highlighted with the red circle above shows a case where the fan shroud has been bent. This happened when the printhead accidentally clashed with a tall print. What happens is that the fan was blowing constantly directly toward the heater block rather than the tip of the nozzle. Continuous cooling of the heater block will make it inefficient in heating up the filament and creating underextrusion.
The image above shows the correct orientation of the fan shroud. note that the flaps are directed towards the tip of the nozzle.