I'm sorry, I seem to have misunderstood or missed something in your first post. If you are referring to the knuckle at the middle of your finger, that will just take time. When you said "2nd knuckle" I thought you meant the striking knuckle of the 2nd finger.
That being said, there are some limitations for some people to get the finger curled enough. If so, try letting the end knuckle of your first finger stay straight and the finger print pad to lay flat on the meat of your thumb/palm. Then curl your thumb over it. It will allow the finger to be more compact without taking much away from the striking knuckle.