Make sure that the latch clicks in when connecting 2 sections of tube. Never try and connect or disconnect the tubes without placing the unit on the ground to take the weight off the coupler and attachment tube.
1) Get them both lined up on the ground and then gently push them together holding the coupling ends of each tube (leaving the cutting head and engine resting on the ground).
This allows the tubes to line up without requiring any effort. If any of the sections are off the ground it is much harder to line them up in a straight line and you get friction or binding as you push them together. The same applies when removing. The 7 teeth on the drive shaft inside the attachment tube have to slide into the female receptacle inside the coupler.
2) Push attachment tube into coupler and gently rotate until you feel the attachment drive shaft teeth engage and slide into the female connector inside the coupler. When you have done this, the tube will slide further into the coupler and the latch on the coupler will "click" into the hole on the attachment tube.
3) Push down the black protective cover and tighten firmly the clamp bolt (black handle)
4) When disconnecting, put the unit on the ground and undo the clamp bolt and the protective cover should flip up.
5) Push down the latch with the tip of your thumb to make sure it goes all the way down. The latch may be hard to push down if the hole in attachment tube is rotated against the latch. Simply rotate attachment tube slightly until it is easy to push down latch.
6) With the unit still on the ground, keep the tubes parallel as your pull apart. The attachment tube drive shafts may slide out of tubes as you separate. This is NORMAL to allow them to be inspected. They can simply be pushed back inside the tubes when storing.