Help with my knuckleball

My brother is trying to learn the knuckleball, he’s pretty good at throwing it with no spin at all but for some reason it wont knuckle. He’s thrown it against me a couple times and it doesnt dance or even wiggle it just looks like a slow pitch with no spin or movement. Do you have any advice I can give my brother.Thanks for your help.

get him to either throw the pitch a little slower/quicker, and get a little spin can also give movement

He needs to get on top of the ball and throw it above 50 mph or even faster to make sure it moves.