Make sure you are installing the bowden tube correctly. Hold the collar down with a fingernail:
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Insert tube till it won't go further.
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Pull collar up with a fingernail while pushing the tube down. You should feel a faint click in the collar when it has been pulled up enough, that is the metal teeth biting into the bowden tube.
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Insert retainer ring, and you shouldn't have a problem with it popping out ever again.
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Edit: Forgot to mention something important: Make sure the end of your tube is cut flat and flush. The flush cutters that come with most 3D printer kits is usually good enough for the job, or you can use a razor blade. The specialty cutters work, but really it just needs to be flat enough to seat properly, so you don't have to buy a specialty tool if you don't want to.