Installing bowden tube into E3D V6
You may need to secure the pneumatic coupling in the closed position with a small plastic clip (which should be supplied with the hot end).



You can print your own, providing that your printer will work for long enough (a paper clip might do the trick):

Thingiverse: Bowden Tube Clip v3

Addendum:

Some pneumatic couplers are sprung, so that you have to depress the coupling ring in order to release the tubing. In this case, no clip is usually required. Other pneumatic couplers (such as the one on an E3D V6) are unsprung. This makes it easier to secure and release the tube using the supplied clip. The disadvantage is that you may lose the clip. Sprung couplers sometimes lose their springiness, in which case a clip can be used to secure them in the closed position

It's "swings and roundabouts", really. You can to choose between the awkwardness of sprung couplers, or the risk of losing a clip. Either way, print some spare clips. You may need them one day.