Do the following if you have underextrusion

1. Check the Extruder

Extruder arms, especially ones made of plastic can crack causing poor extrusion. A cracked extruder arm won't apply enough pressure on the filament to push it through the hot end. Check your extruder for damage and make sure all screws and bolts are snug.

2. Check the Feeder Tension

Look at filament that has passed through the extruder. If it is smooth, the extruder isn't putting enough pressure on the filament. There should be marks on the filament from the extruder gears

3. Check the Filament Diameter

Ensure the filament diameter entered in the slicing program is the same diameter as what you are using.

4. Calibrate E-Steps

If the extruder has not been calibrated, the printer may be moving less filament through the extruder than it thinks it is. A helpful guide on how to calibrate e-steps can be found here.

5. Increase the Flow

Try progressively increasing the material flow rate in increments of 2.5% until better results are achieved.

6. Increase the Print Temperature

If the temperature is too low for the material you are using, not enough material will be pushed through the nozzle. Try progressively increasing the print temperature in increments of 5 degrees until better results are achieved.