<p>I've been printing PLA on a Pegasus 12" for 2 years. Print quality has been great but in the last few months the quality of the prints has deteriorated. After looking into it, it seems when the head moves away from the spool and puts the filament in tension (and pulls on the spool) the extruder cog slips on the filament. 
That results in traces where no plastic gets extruded the furthest away the head moves from the spool (the spool is on the left in the picture below):</p>

<p><a href="https://i.stack.imgur.com/0W9DA.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0W9DA.jpg" alt="See diagonal traces where plastic didn&#39;t extrude"></a></p>

<p>The cog slipping can be seen between 0:22 and 0:27 on the video below: the cog on the right spins continuously but the ball bearing on the left stops spinning for 5 seconds.
<a href="https://photos.app.goo.gl/cTfySUgXDy1XKXGv8" rel="nofollow noreferrer">https://photos.app.goo.gl/cTfySUgXDy1XKXGv8</a></p>

<p>The end result is a part with multiple gaps, especially on the side farthest away from the spool.</p>

<p><a href="https://i.stack.imgur.com/lX4eJ.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lX4eJ.jpg" alt="enter image description here"></a></p>

<p>I've cleaned the teeth of the cog (removed plastic dust) to improve traction but that didn't solve the problem. I've also tried to clean up the extruder by inserting a wire from the hot end side, no improvement in print quality.</p>

<p>Any ideas on how to fix that slipping problem?</p>

<h1><strong>Edit with solution:</strong></h1>

<p>To complement Oscar's answer below, here is what I've done that fixed the problem: it turns out the issue was coming from a lack of friction on the filament. To increase the friction, I've slipped a pair of 0.25mm thick pieces of plastic sheet between the bearing axis and the mount. See pic below.</p>

<p><a href="https://i.stack.imgur.com/CXw36.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CXw36.jpg" alt="enter image description here"></a></p>

<p>That increased the force on the filament enough to fully solve the slippage problem. The print quality is perfect now (see pic below).</p>

<p><a href="https://i.stack.imgur.com/KqbYo.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KqbYo.jpg" alt="enter image description here"></a></p>

<p>The plastic sheet trick is just a short term fix. I'm going to try swapping the drive gear for a slightly thicker one and contact MakerFarm to see if there is any long term modification they recommend.</p>

[{'answer_id': 7182, 'author': 'profesor79', 'author_id': 9730, 'author_profile': 'https://3dprinting.stackexchange.com/users/9730', 'pm_score': 3, 'selected': False, 'text': '<p>The soldering in the Tevo components is very low quality as I replaced/resoldered most of the end-stops.</p>\n\n<p>As the gantry goes down - please ensure that the cable is not pulled over (no contact) and there is contact on the edge of the acrylic and the end-stop, also the small acrylic switch holder could bend/slip a bit. Finally, the sensor connection to the main board could be dragged by wires that are connected to the hot-end.</p>\n\n<p>To validate micro-switch behavior - lift the Z-axis (about 100mm), then set <code>home position</code> and manually trigger the end-stop switch. That shall lift the Z-axis, so then trigger again to see if that works. You will still have a plenty of time to stop the printer if the switch does not work.</p>\n\n<p>If that works, then the reason could be in slipping edge of the black acrylic plate and the micro-switch. Also validate the mechanical connection (if there is a loosening screw (the small on the switch plate)).</p>\n'}, {'answer_id': 7183, 'author': 'Carl Witthoft', 'author_id': 2191, 'author_profile': 'https://3dprinting.stackexchange.com/users/2191', 'pm_score': 2, 'selected': False, 'text': '<p>I\'m not familiar with your particular model, so this may not be directly applicable.</p>\n\n<p>For many printer models, a swing thru thingiverse.com will find you a replacement z-stop switch mount that uses a threaded screw to set the z-height at which the microswitch fires.  It may well be that you simply need to offset that height by a few mm to get your first layer to mate well with the print bed. </p>\n\n<p>Here are three examples, albeit for various models.  <a href="https://www.thingiverse.com/thing:12488" rel="nofollow noreferrer">12488</a>  ,    <a href="https://www.thingiverse.com/thing:1829747" rel="nofollow noreferrer">1829747</a> ,<br>\n  <a href="https://www.thingiverse.com/thing:2856144" rel="nofollow noreferrer">2856144</a></p>\n'}]
['https://3dprinting.stackexchange.com/questions/7188', 'https://3dprinting.stackexchange.com', 'https://3dprinting.stackexchange.com/users/13222/']