Messages on iPhone, iPad and Macbook. One persons messages not on iPad
Messages syncing can be finicky. Go to Settings > Messages > Send &amp; Receive and verify that your iPad is configured to use all the same addresses that your iPhone and Mac are using. 

It is also helpful to make certain that all devices have the same "Start new conversations from" setting. I recommend setting them to start from your phone number, as that's where other are likely to message you. Otherwise you end up with multiple conversation threads with the same person.