The bowden gap issue is a very common problem that occurs on Creality machines and others with similar hotends that can cause under extrusion, weak prints or excessive stringing.
Explanation of the problem: The issue caused by a gap that forms between the bowden tube and the nozzle. In these machines the bowden tube comes out of the extruder and goes all the way through the hot end until it comes in contact with the nozzle. After printing for some time, retractions can cause the tube to back away from the nozzle leaving a gap. The tube can also degrade from the heat or the fitting holding the tube in can fail causing the same problem.
Symptoms of this issue: This issue can cause under extrusion and weak prints, especially near the seam after travel. It can also cause excessive stringing, because the filament is unable to retract properly. If your filament is very difficult to unload even after heating the nozzle, that can be a clue that you have this gap.
In order to fix this we need to reseat the tube tightly against the nozzle.
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Heat the nozzle, unload the filament and then remove the nozzle.
If you see filament on the outside of your tube like this, then you can be sure there was a gap. This is a photo from one of my machines that had this issue:
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Remove the clip from your coupler (see diagram above) and while pressing the release on top of the coupler, push the tube all the way out of the bottom of the heater block.
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Clean any excess filament off the tube. If the tube end is degraded and not cut straight you may need to remove it and cut a section off. You want a straight flat edge so that it can sit flush against the nozzle. There are tools to cut straight or you can print a cutting guide off thingiverse.
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Pull the tube back out so you can reinstall the nozzle.
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With the machine hot (be careful!) reinstall the nozzle all the way. Once it is all the way screwed it, unscrew it one full turn. This will allow us to get a nice tight fit to the tube.
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Insert the tube all the way to the nozzle and reinsert the retaining clip into the coupler.
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Retighten the nozzle all the way. This extra turn while the tube is already touching the nozzle will help seal the gap.
As a test, tug on the tube to make sure it remains tight in the hot end. If the tube can move in and out of the hotend you may need to replace the coupler that holds it in the hot end. You can do the same test on the extruder side.
Here is a great video from Tomb of 3D Printed Horrors where he explains this issue and goes over the steps above: Fixing Ender 3 and Ender 5 Bowden Tube Gap - YouTube