Not sure what you guys are doing, we've never had a problem with ones fitted here I had one on my commuting bike too and it was perfect (only changed it so I could fit gears).
The O Ring is just to hold the bolt in place (push it out and you can see how the system works), the head of the bolt butts up against the main arm of the tensioner to lock the arm against the hanger.
The angle of the arm isn't what is designed to add the tension. In fact, having the arm NOT like in the photo, but tucked up against the chainstay, is the correct way to fit it. If you have it sticking down, then heavy landings will force the arm up towards the frame and loosen the bolt.
Here is how we set them up:
- Undo the pinch bolt for the lower cage (4mm allen key)
- Set the arm up so its just underneath the chainstay (or touching)
- Drop the chain off your front sprocket and let it rest around your BB axle
- Line up the lower cage with your rear sprocket (left-right), then pull the bottom of the lower cage backwards to get the chain taught.
- Nip the pinch bolt up
- Pedal the chain back onto your front sprocket; this adds tension to the spring and you're good to go