The new "flat" keys from both Kryptonite and OnGuard require that the key be inserted ALL the way to the bottom before turning the key. There are about five to eight moveable plates inside the keyway, and if you turn the key before reaching the bottom of the keyway, the plates go out of alignment, and the lock will not open.

This is explained in both the instructions and on the "red" sticker on the locks. If you have misaligned the plates, just shine a bright light into the keyway. Use the key to realign the top plate to the second plate. Then, the second plate to the third plate...When you have ALL of the plates aligned the lock opens easily.

The purpose of this design is to make the lock very difficult to pick. After you have learned how to use the lock properly, you can do easily, without a second of thought.