レスポンシブが上手くいきません。

レスポンシブデザインにしようと思っています。
@media screen and (max-width: 640px)以下を書くと、
header-main-coverが画面の左に飛んで行ってしまいます。
本当はwidth:640px以上ではheader-main-coverを画面の真ん中に写したいのです。
なにが間違っているのでしょうか？

* {
  margin: 0;
  padding: 0;
  -webkit-box-sizing: border-box;
          box-sizing: border-box;
  outline: none;
  font-family: 'Roboto', sans-serif;
}

.header-main {
  height: 80px;
  background-color: black;
}

.header-main-cover {
  max-width: 1080px;
  margin: 0 auto;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: justify;
      -ms-flex-pack: justify;
          justify-content: space-between;
  margin-left: 10px;
}

.header-main .logo {
  float: left;
}

.header-main .logo-img {
  height: 80px;
  margin-left: 10px;
}

.header-main .form {
  margin-top: 10px;
  margin-bottom: 10px;
  margin-right: 10px;
  background: -webkit-gradient(linear, left top, left bottom, color-stop(50%, #ffcc00), to(#ee8f0b));
  background: linear-gradient(to bottom, #ffcc00 50%, #ee8f0b);
  -webkit-transition: all 0.5s;
  transition: all 0.5s;
  width: 200px;
  border: 2px solid black;
  border-radius: 8px;
}

.header-main .form:hover {
  background-color: #ffcc00;
}

.header-main .form a {
  color: rgba(5, 1, 1, 0.842);
  font-weight: bold;
  line-height: 60px;
  display: block;
  text-align: center;
  text-decoration: none;
}

@media screen and (max-width: 640px) {
  .header-main {
    height: 80px;
    background-color: black;
  }
  .header-main-cover {
    max-width: 640px;
    margin: 0 auto;
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;
    -webkit-box-pack: justify;
        -ms-flex-pack: justify;
            justify-content: space-between;
    margin-left: 10px;
  }
  .header-main .logo {
    float: left;
  }
  .header-main .logo-img {
    height: 60px;
    margin-left: 10px;
  }
  .header-main .form {
    margin-top: 10px;
    margin-bottom: 10px;
    margin-right: 10px;
    background: -webkit-gradient(linear, left top, left bottom, color-stop(50%, #ffcc00), to(#ee8f0b));
    background: linear-gradient(to bottom, #ffcc00 50%, #ee8f0b);
    -webkit-transition: all 0.5s;
    transition: all 0.5s;
    width: 200px;
    border: 2px solid black;
    border-radius: 8px;
  }
  .header-main .form:hover {
    background-color: #ffcc00;
  }
  .header-main .form a {
    color: rgba(5, 1, 1, 0.842);
    font-weight: bold;
    line-height: 60px;
    display: block;
    text-align: center;
    text-decoration: none;
  }
}
/*# sourceMappingURL=style.css.map */

<section class="header-main">
      <div class="header-main-cover">
        <div class="logo">
          <img class="logo-img" src="img/logo.png" alt="会社ロゴ">
        </div>
        <div class="form">
          <a href="#">問い合わせフォーム</a>
        </div>
      </div>
    </section>