What would the process be to fix my cracked fake front tooth? What has caused this to happen?

I broke off half of my front tooth in January of this year. I went to the dentist and he gave me a root canal and also shaved down the rest of my tooth. He rebuilt my tooth and told me that the fake tooth should last forever. It is now November and I have two small cracks in the tooth at my gum line. Most people I know with the same treatment I have also have small cracks or have lost the fake tooth (we have different dentist). If it withstands force and lasts forever why is it messed up?