The eruption time for permanent teeth has passed but they haven't come in.

If more than 6 months have passed since the baby tooth came out with no permanent tooth appearing, or if it is more than 6 months behind the same tooth on the other side, you should have an X-ray. The causes range from 1) gum tissue too thick for the tooth to break through, to 2) a permanent tooth positioned at an angle so its path is blocked, to 3) rarely, congenital absence of the tooth bud itself. An X-ray shows whether the permanent tooth exists and where it is, so we can judge whether to wait or to guide it into place. Orthodontic treatment becomes more complex if the right window is missed, so it is best to check without delay.