Less plastic leaves the nozzle than the slicer asked for, so lines come out thin and layers do not bond.
Gaps between lines, thin or missing walls, and a print that feels light and snaps easily. It is easy to miss at first. The printer cannot tell how much plastic actually leaves the nozzle, so it carries on printing as if nothing is wrong while every layer comes out a little starved.
If the nozzle sits too close to the bed, plastic cannot get out. Recalibrate the first layer after moving the printer, changing the nozzle or swapping build plates.
Do a cold pull or follow your printer maker's unclogging guide. Push filament through by hand at printing temperature; it should come out smooth and straight down.
A nozzle refitted wrongly can clog or leak. Hardened or stainless steel nozzles often need 5 to 10 °C more than brass, according to Prusa.
Dust builds up on the drive gears. Clean it off and set the idler tension your printer's maker specifies, since too loose slips and too tight grinds the filament.
Use the temperature your slicer's profile sets for the filament. Dry damp filament, and try another spool if the diameter looks uneven.
Once the hardware is right, calibrate e-steps, then flow ratio for each filament.
Prusa's under-extrusion guide covers each of these checks for its printers, and Bambu Lab's clog guide covers extruder and nozzle clogs on Bambu printers.