ngIf doesn't evaluate the expression when it changes. Angular and Firebase

First I set the isLoading property to "true" so the element isn't loaded yet.(this happens on AppComponent)

  export class AppComponent implements OnInit {
  isLoading = true;
  constructor(private authService: AuthService) {}
 .
 .
 .

bellow in the ngOnInit hook I have:

firebase.auth().onAuthStateChanged((user) => {
      if (user) {
        // User is signed in.
        this.authService.user = new User(user.displayName, user.email, user.photoURL, user.uid);

        console.log(user);
        this.isLoading = false;
        this.authService.isLogIn.next(true);
      } else {
        // No user is signed in.
        console.error('no user is login');
        this.authService.isLogIn.next(false);
      }
    });
    .
    .
    .

this code is an asynchronous one and I want that the user wait to check if him is login or not so I can change the header's UI. Besides I have another component that wraps the header. I call it inside the AppComponent with *ngIf=isLoading

<div>
  <app-header *ngIf="!isLoading"></app-header>
</div>


<div>
  <router-outlet></router-outlet>
</div>

The problem is that when the isLoading change its value to false I've never seen the AppHeader been rendered. So, why *ngIf doesn't evaluate that property again?

Here my AuthService:

import * as firebase from 'firebase';
// import {User} from "../../auth/models/user.model";
import {Router} from '@angular/router';
import { Injectable } from "@angular/core";
import {Subject} from "rxjs/Subject";
import {User} from "../../auth/models/user.model";

@Injectable()
export class AuthService {
  token: string;
  isLogIn = new Subject<boolean>();
  user: User;
  constructor(private router: Router){}

  signinWithFacebook() {
    const provider = new firebase.auth.FacebookAuthProvider();
    provider.addScope('user_location');
    return firebase.auth().signInWithPopup(provider)
      .then(
        (res) => {
          console.log(res);
          this.getTokenId();
          localStorage.setItem('userInfo', JSON.stringify(firebase.auth().currentUser.providerData[0]));
          const userInfo = firebase.auth().currentUser.providerData[0];
          this.user = new User(userInfo.displayName, userInfo.email, userInfo.photoURL, userInfo.uid);

          this.isLogIn.next(true);
          this.router.navigate(['/dashboard']);
        }
      );
  }

  getTokenId() {
    firebase.auth().currentUser.getIdToken()
      .then(
        (tk) => {
          return this.token = tk;
        }
      );
  }

  logout() {
    return firebase.auth().signOut();
    // handle in component
  }

  getLocalUserInfo(): boolean {
    if(localStorage.getItem('userInfo')) {
      const transformStoredUser = JSON.parse(localStorage.getItem('userInfo'));
      this.user = new User(transformStoredUser.displayName, transformStoredUser.email, transformStoredUser.photoURL, transformStoredUser.uid);
      return true;
    } else {
      return false;
    }
  }
  isLogin():boolean {
    if (localStorage.getItem('userInfo')) {
      return true;
    }
    return false;
  }
}

AppHeader.ts

import { Component, OnInit } from '@angular/core';
import {AuthService} from '../../public/services/auth.service';
import * as  firebase from 'firebase';
import {User} from "../../auth/models/user.model";
import {Observable} from "rxjs/Observable";

@Component({
  selector: 'app-header',
  templateUrl: './header.component.html',
  styleUrls: ['./header.component.css']
})
export class HeaderComponent implements OnInit {
  isActivated = false;
  constructor(private authService: AuthService) { }
  ngOnInit() {
    this.authService.isLogIn.asObservable().subscribe(
      (data: boolean) => {
        this.isActivated = data;
      }
    );
    this.isActivated = this.authService.isLogin();
  }

}

AppHeader template

<nav class="navbar navbar-expand-lg navbar-dark bg-custom-nav px-5">
  <a class="navbar-brand" routerLink="/">MovieApp</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNavAltMarkup" aria-controls="navbarNavAltMarkup" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="container-fluid">
    <div class="collapse navbar-collapse d-lg-flex justify-content-lg-between row" id="navbarNavAltMarkup">
      <div class="navbar-nav px-sm-4 px-lg-0">
        <a class="nav-item nav-link text-white"
           routerLink="/"
           routerLinkActive="active font-weight-bold"
           [routerLinkActiveOptions]="{exact: true}">Inicio</a>
      </div>
      <div class="col-lg-8">
        <app-search-bar></app-search-bar>
      </div>
      <div class="row">
        <ng-template [ngIf]="!isActivated">
          <a class="nav-item nav-link text-white anchor-hover"

             routerLink="/login"
             routerLinkActive="active font-weight-bold">
            <i class="fa fa-sign-in pr-2" aria-hidden="true"></i>Ingresar
          </a>
        </ng-template>
        <ng-template [ngIf]="isActivated">
          <!--DROPDOWN PROFILE-->
          <div class="btn-group" role="group" *ngIf="authService.user">
            <button id="btnGroupDrop1" type="button" class="btn btn-light dropdown-toggle pointer" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true">
              {{authService.user.displayName}}
            </button>
            <div class="dropdown-menu dropdown-menu-right" aria-labelledby="btnGroupDrop1">
              <a class="dropdown-item" href="#">Dropdown link</a>
              <a class="dropdown-item" href="#">Dropdown link</a>
            </div>
          </div>
          <a class="nav-item nav-link text-white anchor-hover"
             routerLink="/dashboard"
             routerLinkActive="active font-weight-bold">
            <i class="fa fa-sign-in pr-2" aria-hidden="true"></i>Dashboard
          </a>
        </ng-template>

      </div>
    </div>
  </div>
</nav>