Blender material prob

Hey everyone i have a quick question. i am new to blender and i made my first human model it preety good (for me) but when i joined the head and the body i tried to add a texture. the texture works on the ears and the body but not the head. i figured out if i delete a face on the head then re-enter it it will have the applied matirial. ive tried reloading and even in the render it does it. is there a way to reauthenicate the faces or will i have to make each face all over again?

You can apply numerous materials to a single mesh object in the Editing (F9) menu. Click “new” to add a new material, select the faces you want the material to be on, then click “assign.”