The most common reason a permanent tooth does not erupt through the gums is because there isn’t enough space for it. Sometimes nearby teeth need to fall out before the permanent tooth will come up. There are some rarer cases where a primary (baby) tooth is lost earlier than it should be (potentially due to an injury) which can explain why the adult tooth hasn’t come-in. Even rarer, ankylosis is a condition where teeth cannot erupt because they are stuck to the jawbone. Generally the best thing to do is wait, and if you are worried about a tooth not coming in, talk to your dentist.