$_POST无法处理某些字段

我正在处理PHP的电子邮件，其中5个表单字段将重试并在邮件中发送。我得到了一些代码来实现这一点，其中代码只为4个文本字段编写，而我有5个文本字段。问题是$_POST没有检索第五个字段值，即使我将其按不同的顺序放置，它也没有检索值。以下是代码和结果。

letusknow.php文件：

<div class="row">
            <form id="enquiryForm" class="form-horizontal" novalidate>
                <fieldset>
                <!-- Form Name -->
                <legend>Free Quote</legend>
                <!-- Text input-->
                <div class="form-group">
                  <label class="col-md-2 control-label">Name</label>  
                  <div class="col-md-4">
                  <input id="name" name="name" type="text" placeholder="Enter your name" class="form-control input-md" required data-validation-required-message="Please enter your Name.">
                  </div>
                </div>
                <!-- Text input-->
                <div class="form-group">
                  <label class="col-md-2 control-label">Email</label>  
                  <div class="col-md-4">
                  <input id="email" name="email" type="email" placeholder="Enter your email ID" class="form-control input-md" required data-validation-required-message="Please enter your Email Id.">
                  </div>
                </div>
                <!-- Text input-->
                <div class="form-group">
                  <label class="col-md-2 control-label">Mobile</label>  
                  <div class="col-md-4">
                  <input id="phone" name="phone" type="tel" placeholder="Enter your mobile number" class="form-control input-md" required data-validation-required-message="Please enter your Phone Number.">
                  </div>
                </div>
                <!-- Text input-->
                <div class="form-group">
                  <label class="col-md-2 control-label">City</label>  
                  <div class="col-md-4">
                  <input id="city" name="city" type="text" placeholder="Enter your city" class="form-control input-md" required data-validation-required-message="Please enter your City.">
                  </div>
                </div>
                                <!-- Prepended text-->
                <div class="form-group">
                  <label class="col-md-2 control-label">Product Loan Amount</label>
                  <div class="col-md-4">
                    <div class="input-group">
                      <span class="input-group-addon">(INR) ₹</span>
                      <input id="productLoan" name="productLoan" class="form-control" placeholder="Enter product loan amount" type="number" required data-validation-required-message="Please enter the Product Loan Amount.">
                    </div>
                  </div>
                </div>
                <!-- Multiple Checkboxes -->
                <div class="form-group">
                  <label class="col-md-2 control-label" for="checkboxIAuthorize"></label>
                  <div class="col-md-4">
                  <div class="checkbox">
                    <label for="checkboxIAuthorize-0">
                      <input type="checkbox" id="checkboxIAuthorize-0" value="">
                      I authorize to contact me
                    </label>
                    </div>
                  </div>
                </div>
                <!-- For success/fail messages -->
                <div id="success"></div>
                <!-- Button -->
                <div class="form-group">
                  <label class="col-md-2 control-label"></label>
                  <div class="col-md-4">
                    <button name="submit" type="submit" class="btn btn-primary">Submit</button>
                  </div>
                </div>
                </fieldset>
                </form>
        </div>

enquiry_mail.php文件：

<?php
$name = $_POST["name"];
$email_address = $_POST["email"];
$phone = $_POST["phone"];
$city = $_POST["city"];
$loan = $_POST["productLoan"];
// create email body and send it    
$to = 'support@mysampledomain.com'; // PUT YOUR EMAIL ADDRESS HERE
$email_subject = "Enquiry Contact Form:  $name"; // EDIT THE EMAIL SUBJECT LINE HERE
$email_body = "You have received a new enquiry from your website's Enquiry form.'n'n"."Here are the details:'nName: $name'nPhone: $phone'nEmail: $email_address'nCity: $city'nProduct Loan Amount: $loan";
$headers = "From: $email_address'n";
$headers .= "Reply-To: $email_address"; 
mail($to,$email_subject,$email_body,$headers);
return true;            
?>

我执行后，邮件被发送到support@mysampledomain.com结果如下。

You have received a new enquiry from your website's Enquiry form.
Here are the details:
Name: TestName
Phone: 9191919191
Email: testname@testsample.com
City: Test City
Product Loan Amount: 

此处不使用值重试产品贷款金额。$_POST失败。帮我解决这个问题。