移动设备无法向下滚动到模式的底部？

Question

当我在移动设备上访问以下页面时，我只能向下滚动直到无序列表中的最后一个元素。如果我试图向下滑动更多，它就不会移动。我无法从PayPal和/或表单底部看到“购买”按钮。为什么会这样，我该如何解决？

    <xen:if is="!{$visitor.user_id}">

    <style>

    .modal {
    display: none;
    position: fixed;
    z-index: 9990;
    padding-top: 100px;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    overflow: auto;
    background-color: rgb(0,0,0);
    background-color: rgba(0,0,0,0.4);
    }
    .modal-content {
    background-color: #fefefe;
    margin: auto;
    padding: 30px 30px 10px 30px;
    border: 1px solid #888;
    border-radius: 30px;
    width: 50%;
    font-size: 14pt;
    }

    .modal-content h3 {
    font-weight: 700;
    font-size: 1.6em;
    margin-bottom: 20px;
    text-align: center;
    }
    .modal-content ul {
    margin: 10px 25px 20px;
    }
    .modal-content li {
    list-style: inherit;
    line-height: 1.5;
    }
    .closeSubscribe {
    display: hidden;
    color: #aaaaaa;
    float: right;
    font-size: 28px;
    font-weight: bold;
    }

    .listitem {
    margin-top:10px;}

    .closeSubscribe:hover,
    .closeSubscribe:focus {
    color: #000;
    text-decoration: none;
    cursor: pointer;
    }
    #subscribeForm input[type=image] {
    display: block;
    margin: 0px auto;
    }
    </style>

    <!-- The Modal -->
    <div id="subscribeModal" class="modal">

      <!-- Modal content -->
      <div class="modal-content">
      <h3>Become a Member Today For £9.99!</h3>
      <p style="margin-bottom:10px;">And:</p>
      <ul>
      <li class="listitem">x</li>
      <li class="listitem">y</li>
      <li class="listitem">z</li>
      <li class="listitem">a</li>
      <li class="listitem">b</li>
      <li class="listitem">c</li>
      </ul>
      <form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_top" id="subscribeForm">
    <input type="hidden" name="cmd" value="_xclick">
    <input type="hidden" name="business" value="a@gmail.com">
    <input type="hidden" name="lc" value="US">
    <input type="hidden" name="item_name" value="an">
    <input type="hidden" name="amount" value="9.99">
    <input type="hidden" name="currency_code" value="GBP">
    <input type="hidden" name="button_subtype" value="services">
    <input type="hidden" name="no_note" value="0">
    <input type="hidden" name="bn" value="PP-BuyNowBF:btn_buynowCC_LG.gif:NonHostedGuest">
    <input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_buynowCC_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
    <img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
    </form>
  </div>
    </div>

    </xen:if>