Try to align the hex nut into the hex socket and then screw the knob in.

 

Didnt work, for some reason. It does kinda screw in, and it does align, but it doesnt screw in fully.
Its either that the hex itself is faulty, or i might have to force the screw in

 

How did the know come out to begin with? Did it unscrew as you were adjusting the knob?
Did it take a bit of force?