I own an Ender 3 that I bought in December 2018. I have now a problem with under extrusion that I don't know how to solve. I tried everything: I changed the nozzle, changed the PTFE, changed the filament, dismantled the hotend and controlled that nothing was blocking the flow. The problem is that it isn't a normal under extrusion: in fact, the printing starts quite well, then for some reason the filament doesn't flow anymore. If I try to push the filament I feel like there is a gap that, if I continue pushing, it's "filled" and then the plastic continues to flow normally for some time. And when it has to do very little parts of the printing, this problem occurs, and the final result is a disaster. I hope someone can help me because I don't know what to do. The photos show the condition of the printing, and I have also a brief video that explains the situation better. I added also an image of the original CAD file, so you can see how it should have been, and another project where you can see missing layers when things get "difficult" to print (but, in the past, the printer worked perfectly with such projects!) and a simple parallelepiped, that the printer printed perfectly (I assume it's because it never stops sending filament, I don't know). Thank you for your time.
DropBox link for the photos: https://www.dropbox.com/sh/xav38c8tw8mgs8h/AAA7VGypWIrIR0RwB4Ih3O7ma?dl=0
YouTube link for the video (I recorded the printer printing and made a video with the most important moments such as when there is under extrusion; the video lasts 4 minutes and it can help you find the problem):