This article concerns the ticking noise that can be made by the Ultimaker 2, Ultimaker 2 Extended, and Ultimaker 2Go running the standard black box feeder. While the Ultimaker 2+ family has a different feeder and does not make a ticking noise, you may still find this article useful for tracking the cause of under extrusion.
My printer is making a ticking noise when it prints. It's under extruding and the feeder is moving backwards sometimes. What's wrong with my printer?
The ticking itself is pretty straight forward--it means the filament is encountering resistance of some form while it tries to print. If it encounters anything that's causing resistance, the motor will turn back slightly and try again to avoid chewing up the same spot of filament.
There are a number of things that can cause resistance/skip-back. Here are the main points on your filament feed path that could be causing it:
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Filament--if the filament is oversized, tangled, or catches on anything, you can start seeing skip-back. Also, as you get near the end of a reel of filament, the curvature of the filament is a lot tighter. That in itself is going to create more resistance. You can counter it by trying to straighten it out or try raising the temperatures, but the motor is going to have to work a lot harder to feed it when it's more tightly coiled, and you could see under extrusion throughout the print, or see the lines in the top of the print not quite closing. This is pressure related under extrusion. Another alternative would be to use a filament that doesn't come on a spool/reel. They've got a much wider curvature all around and it's consistent throughout the entire length of the filament, so the motor isn't working as hard. Using a non-spooled filament can take up more space in your set up, but your feeder won't have to work as hard since it's not tightly coiled.
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Bowden Tube--The bowden tube is soft and can get a lot of wear and tear. How soon your bowden tube needs to be replaced depends a lot on the materials you're using and how much retraction is involved. You can end up with the tube getting chewed up at the feeder end, or end up with rough spots inside the tube. To check for rough spots inside the tube, take both ends out of the printer (slide the blue clip out, and push down on the white collet while pulling up on the tube) and slide a fresh piece of filament through the tube by hand. If there's any point where you feel resistance, then there's damage in the tube and it should be changed out. If the end of the tube is chewed up, then you should be able to cut the end off and widen it out again with an exacto knife and it should still be usable.
The other thing that can cause resistance in the tube is if there are ground up bits of filament in it. If there are, you can wad up a small piece of paper towel and put it into the end of the tube, and push it all the way through with a piece of filament. This should gather up all the debris and clean it out. 
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PTFE Coupler--The PTFE coupler has a lifespan of about 500 print hours with PLA, though this lifespan can be affected by variables such as the amount of retraction in your prints and whether you're using abrasive materials. Due to the filament passing through and the exposure to the heat of the nozzle, the coupler deforms over time and you can end up with a lip on the inside that the filament can catch on, and end up with hole widened out, which can make the filament widen out and not feed well. Below are 2 particularly bad PTFE couplers along side a new PTFE coupler for comparison.
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Nozzle--If you've recently had a print fail where the nozzle would have stayed hot for a while without filament moving through it, you can end up with burnt material in the nozzle, which would slow down the flow. Similiarly, if you've flipped between PLA and a high temperature material (ABS, CPE, FlexPLA, etc.) you can end up with bits of the other kind of material stuck in the nozzle and impeding the flow. Cleaning the nozzle with the Atomic Method is the best way to get the nozzle clean. When you do the cold pull on the Atomic Method, you really need to see a clean imprint of the nozzle (it should like the tip of a crayon). If the nozzle impression doesn't look like that, you're not done with the Atomic Cleaning yet. It could take 10 or 15 tries if it's really burnt on there well. If you've been working with both a high temperature filament and PLA, you'll need to use the higher temperature filament and higher temperature instructions for the cleaning. For more information on filament types, see our Filament FAQ.
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Feeder--The feeding mechanism on the UM2 is the other place to keep an eye on. If you've got ground up material on the feeder, you're going to need to clean it out with canned air (pull the bowden tube out first so you're not spraying the debris up into the tube). The filament will slip if too much ground material is on the knurled sleeve. The UM2 feeder is also a little sensitive. In addition to the tension screw that controls the spring, the 4 screws that hold the feeder housing on play a huge roll in how well the filament feeds. If those 4 screws are too tight, the filament will skip. Make them hand tight, and then loosen them a 1/4 turn. You can try loosening it a little more if it's skipping at that tension.
- Temperature--Different types of filament (and even different colors within the same type or brand) can have varied responses to temperature, due to the different chemical makeup. Raising the temperature 5-10 degrees is a good way to check your extrusion. If there is something causing resistance, increasing the heat can help overcome it. If the room is too cold, there's a vent blowing on your printer, or your fan shroud is touching your nozzle, the temperature of your printer can be affected. Temperature plays a vital role in printing with any type of plastic. You can get a smoother, more reliable flow by raising the temperature, but you can also see more stringing with higher temperatures because you're dealing with molten plastic.
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