Here is a relatively quick drill every Maker of 3D Printing will need to know. Blocked hotends, also known as under extrusion, plugged, jammed, or mucked, can happen from poor quality filament but, often happen because of movement of PTFE away from the nozzle creating a void. This causes a plug to build up and when your printer retracts, it pulls back successfully but not enough to fill the void and extrude filament.
Symptoms are: stringing, hairy prints, layers not connecting tightly, build up of material on the nozzle.
I have a full write-up on makersteve.com here: https://bit.ly/2FGLhYW
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How this video was done:
This was filmed with Windows 10 “Camera” and absolutely no editing.
Logitech HD Pro Webcam C920 https://amzn.to/2ROYQqh
Amazon Basics Tripod https://amzn.to/2DA81b0
Monoprice 32ft Active Repeater Cable https://amzn.to/2T5miRR