Cel shading shadow doesn't show

I use two matereal in one object and use the inverse hull mathot to get an out line

but for some reason, there’s no shadow on the head part, unlike the other part, which has a shadow
So i tried to separate the object into separate material, but it still doesn’t cast any shadow

And the weird part is when I pose the model, the outline somehow casts a shadow but not the object itself.

Is there any way I can fix this?