[PHP Email Forms - doesn't send the email every time]

Bit of a strange problem - I have created two identical websites on different hosts and they both have 3 email forms which do work but not always.  I have tested them 5 times today and only recieved emails from them on 3 occasions.

The two websites affected are

www.fluidstudioswms.com and www.hertfordshire-web-design.co.uk

I am not very good at PHP but can't see anything wrong so it may not be a code problem at all but not sure what else to try.  The code for one of the three forms is as follows:


HTML:
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<form method="POST" action="callback.php">

<input type=hidden name="recipient" value="mail@fluidstudioswms.com">
<input type=hidden name="subject" value="Call back">

                            <p><font size="2">Your name:</font><br>

                            <input type="text" name="q1">

                            <p><font size="2">Telephone number:</font><br>

                            <input type="text" name="q2">       
                 
                            <p><font size="2">Preferred time/day for us to call:</font>
                              <em><font size="2">(9.00 to 5.30 - monday to friday)</font></em><br>

                              <input type="text" name="q3">

                            <p><font size="2">Interested in:<em> (main interest
                              if more than one)</em></font><br>

                              <select name="q4">

                                <option value=webdesign selected>Web Design</option>
                                <option value=printing>Printing</option>
                                <option value=seo>Search Engine Optimisation</option>
                                <option value=other>Other</option>
   
                              </select>

                            <p>
<input type="submit" name="submit" value="Submit">
</form>
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PHP
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<?php


// get data
$EmailFrom = "mail@fluidstudioswms.com";
$EmailTo = "mail@fluidstudioswms.com";
$Subject = "Call back";
$q1 = Trim(stripslashes($_POST['q1']));
$q2 = Trim(stripslashes($_POST['q2']));
$q3 = Trim(stripslashes($_POST['q3']));
$q4 = Trim(stripslashes($_POST['q4']));


// validate

$validationOK=true;
if (!$validationOK) {
  print "<meta http-equiv=\"refresh\" content=\"0;URL=thanks.html\">";
  exit;
}

// prepare body text
$Body = "";
$Body .= "call back! ";
$Body .= "\n";
$Body .= " ";
$Body .= "\n";
$Body .= "Q1. = ";
$Body .= $q1;
$Body .= "\n";
$Body .= " ";
$Body .= "\n";
$Body .= "Q2. = ";
$Body .= $q2;
$Body .= "\n";
$Body .= " ";
$Body .= "\n";
$Body .= "Q3. = ";
$Body .= $q3;
$Body .= "\n";
$Body .= " ";
$Body .= "\n";
$Body .= "Q4. = ";
$Body .= $q4;
$Body .= "\n";
;





// email
$success = mail($EmailTo, $Subject, $Body, "From: $EmailFrom");

// redirect to success page
if ($success){
  print "<meta http-equiv=\"refresh\" content=\"0;URL=thanks.html\">";
}
else{
  print "<meta http-equiv=\"refresh\" content=\"0;URL=problem.html\">";
}
?>
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