One could argue that Phasma gave in because she was scared of Han & Chewie. Plus, she's overconfident and believes it won't do them any good. The real question is why they didn't raise the shields again when they noticed.

Besides several flawed answers, this is a decent article. There’s a book about Finn’s childhood, by the way. It’s silly to think that Ren is lying about the map; as a successor state to the Empire, the First Order has access to many Imperal archives. And nothing Kylo did is new; Vader could stop blaster bolts…