`componentDidMount()` function is not called after navigation

I am using stackNavigator for navigating between screens. I am calling two API's in componentDidMount() function in my second activity. When i load it first time, it gets loaded successfully. Then i press back button to go back to first activity. Then, if i am again going to second activity, the APIs are not called and I get render error. I am not able to find any solution for this. Any suggestions would be appreciated.