The flow regulating valve has two different means of adjustment. The primary flow control is the white knob that you describe. When this knob is fully unscrewed, the valve allows full flow through the radiator. As the knob is tightened, it gradually reduces the flow.
Underneath the knob is a dial numbered 1 to 6. This dial controls the maximum flow through the radiator. Align the number 6 with the slot in the valve body to allow the maximum flow. Details can be found in the installation manual on page 17.