The layer thickness is too big or printing speed is too high (the nozzle temperature  needs to do responded balance to have a faster printing speed) or the temperature is  too low; Or the wall thickness is too thin; Or 3D model is not completely solid;  Or the feeding device’s fixing screw is too loose; Or wrong choice of filament diameter;  Or filament quality is poor; Or the filament gets stuck and could not be fed smoothly.