Under-extrusion happens when the 3D printer fails to supply the amount of plastic required by the slicing software. This leads to gaps between layers, thin walls, or brittle parts. Unlike a full clog, the printer is still functioning, but working inefficiently. Solving it requires checking the entire filament path, from the spool to the software.
The first place to look is the extruder assembly. If the tension on the extruder arm is too low, the gear will slip. If it's too high, the gear can grind the filament. Software and calibration also play a huge role; every printer has "E-steps," which tells the motor how many pulses are needed to move exactly one millimeter of filament.
Finally, we must consider the physical limitations of the hotend. Every hotend has a maximum "volumetric flow rate"—the total volume of plastic it can melt per second. If you attempt to print at very high speeds, you might exceed the heater's ability to melt the plastic, leading to back-pressure and under-extrusion.