When cooking a lobster, you should always start with fresh or completely thawed lobster. If you are trying to cook frozen, a lot of times it will appear done based on the appearance of the outside shell and outside of the meat, but the inside will be ice cold. From there you can’t just cook it until the center is hot because you will over cook the rest of it and make it mush. NOT a good look. Lobster is super expensive so you need to watch it very carefully and pay attention to cooking temperatures and cooking times.