Can't parent teeth to head bone when the teeth are joined to the body, rigging

Hello, everyone. I am having trouble parenting the teeth to the head bone. The teeth and the body are joined and i have tried many times to parent the whole mesh to the bones with automatic weights and every time its the teeth that get left out. The only thing that seems to work is me separating the teeth from the mesh and then parenting the teeth to the head bone manually. But when I try to join the teeth to the mesh again the teeth get left out again. I have also tried to go into weight paint and get the teeth red but i can’t get it to turn completely or even mostly red. Also i’m not sure if that would be the best way. The first pic is what the teeth do when joined to the body and i try and move the head bone . The second pic is of what it looks like when I separated the teeth and parented them to the head bone manually. Any help and advice would be appreciated. Thank you