Under extrusion may caused by:
1) PTFE tube worn out
Replace PTFE tube will solve the problem, we suggest to replace both PTFE tube and nozzle together. As you may need to clean the internal of the heatsink and the heating block. You can purchase the related parts at our shop.
2) Clogged nozzle
Dust on filament, bad quality filament, printing different temperature filament may clog the nozzle. Replace nozzle will help.
3) Extruder arm broken
Extruder arm broken can cause the pressure lost on the extruder, pressure inconsistent will lead to under extrusion.
4) Filament thickness inconsistent
This is quite rare, unless you are using very bad quality filament.
5) Slicing setting issue
Check the flowrate on your slicing setting. Make sure the temperature are set according to the filament manufacturer suggested.
6) Extruder gear worn out
Replace one will help.
Share with us how do you fix this issue in the comments.