QuadPod robot

I’ve just started learning how to apply certain bones to specific objects, currently I’m trying to understand inverse kinematics. My robot is a very basic 4-legged Mech and each leg consists of only 2 bones (Let’s call them the shin and thigh bone) I’ve already made a duplicate bone to control the shin and thigh. When I move the duplicate bone, both the shin and thigh bone move as well, however the “knee” isn’t bending at all for some reason.

So baically my question is, why isn’t the knee bending?