If you find that not enough filament comes out of the nozzle when you print, which results in your print object getting holes or completely missing some layers, then it is probably due to one of two things:
A) Filament gear. If your printer's filament gear that pulls the filament into the printer is worn, dirty, or not fitted properly, and therefore does not make strong enough contact with the filament you have inserted into the printer, it may not be extruded enough. filament during printing. Investigate if your filament gear can be the explanation by doing as in the first half of this video.
B) Nozzle. It is usually the filament gear that is the problem - so DO NOT start cleaning the nozzle until you have tried to change the filament gear. It is relatively difficult to clean the nozzle and improper assembly of the system can aggravate the problem. If you have changed the filament gear, checked that it is assembled correctly and checked that there are no filament residues around the gear and that filament still does not pass through, then continue to the second half of this video - Follow the video slavishly and make sure to assemble the system exactly as instructed in the video.