(Ruby on Rails) Bootstrapが上手く動かない

RailsでBootstrapを導入しようとしているのですが、上手く動きません。
色々試したのですが、エラーが出たり、読み込むクラスと読み込まないクラスがあったり、またサイトによってjqueryを入れたり、入れなかったり、application.scssに@import "bootstrap-sprockets";を入れたり入れなかったりしていて、色々なパターンで試したのですが、上手く動きません。（なぜかclass="btn btn-primary"は反映されるのにclass="navbar-default"は反映されないということが起きたりしました。）
混乱してよくわからなくなってしまったので、アドバイス頂けると幸いです。
よろしくお願いします。

環境
Mac OS high sierra 10.13.6
ruby 2.5.1
rails 5.1.6

htm
1<!DOCTYPE html>
2<html>
3	<head>
4		<meta charset="utf-8">
5		<title>AwesomeEvents</title>
6		<%= stylesheet_link_tag "application", media: "all", "data-turbolinks-track" => true %>
7		<%= javascript_include_tag "application", "data-turbolinks-track" => true %>
8		<%= csrf_meta_tags %>
9	</head>
10	<body>
11		<header class="navbar navbar-default">
12			<button class="btn btn-primary" type="button" name="button">button</button>
13	</body>
14</html>

#gemfile

source 'https://rubygems.org'

git_source(:github) do |repo_name|
  repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
  "https://github.com/#{repo_name}.git"
end


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.1.6'
# Use sqlite3 as the database for Active Record
gem 'sqlite3'
# Use Puma as the app server
gem 'puma', '~> 3.7'
# Use SCSS for stylesheets
gem 'sass-rails'
gem 'bootstrap', '~> 4.1.1'
gem 'bootstrap-sass'
gem 'jquery-rails'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby
gem 'slim-rails'
gem 'html2slim'
gem 'omniauth'
gem 'omniauth-twitter'
# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.2'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.5'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
end

group :development do
  # Access an IRB console on exception pages or by using <%= console %> anywhere in the code.
  gem 'web-console', '>= 3.3.0'
  gem 'listen', '>= 3.0.5', '< 3.2'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

scss
1#application.scss
2@import "bootstrap";
3
4/*
5 * This is a manifest file that'll be compiled into application.css, which will include all the files
6 * listed below.
7 *
8 * Any CSS and SCSS file within this directory, lib/assets/stylesheets, or any plugin's
9 * vendor/assets/stylesheets directory can be referenced here using a relative path.
10 *
11 * You're free to add application-wide styles to this file and they'll appear at the bottom of the
12 * compiled file so the styles you add here take precedence over styles defined in any other CSS/SCSS
13 * files in this directory. Styles in this file should be added after the last require_* statement.
14 * It is generally better to create a new file per style scope.
15 *
16
17 */
18

javascript
1#application.js
2
3
4// This is a manifest file that'll be compiled into application.js, which will include all the files
5// listed below.
6//
7// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
8// vendor/assets/javascripts directory can be referenced here using a relative path.
9//
10// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
11// compiled file. JavaScript code in this file should be added after the last require_* statement.
12//
13// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
14// about supported directives.
15//
16//
17//= require rails-ujs
18//= require jquery
19//= require bootstrap-sprockets
20//= require turbolinks
21//= require_tree .
22
23