PayPal RefundTransaction API (NVP) Issue - php

Question

I'm trying the RefundTransaction API with nvp as [METHOD] => RefundTransaction
Only Common Response Fields returning as a response to PayPal RefundTransaction API Operation (NVP)
Here is the request
Array
(
[METHOD] => RefundTransaction
[VERSION] => 54.0
[USER] => abc***xyz.com
[PWD] => asd******asd
[SIGNATURE] => T535*****************************************qWE3
[CURRENCYCODE] => AUD
[TRANSACTIONID] => 1D4*******40913KKJ
)
Here is the respose I'm getting.
Array
(
[TIMESTAMP] => 2014-11-11T11:40:57Z
[CORRELATIONID] => 3dba******fce
[ACK] => Success
[VERSION] => 54.0
[BUILD] => 13***072
)
The thing is I'm getting the response as Success, but only the common fields are there. Please help
This is how my function looks like
private function nvpCall($transaction_id){
$params['TRANSACTIONID'] = $transaction_id;
$params['REFUNDTYPE'] = 'Full';
$nvpArr = $params ;
$nvpreq = array();
$nvpreq['METHOD'] = urlencode($methodName);
$nvpreq['VERSION'] = urlencode('54.0');
$nvpreq['USER'] = urlencode($this->pp_pro_username);
$nvpreq['PWD'] = urlencode($this->pp_pro_password);
$nvpreq['SIGNATURE'] = urlencode($this->pp_pro_signature);
$nvpreq['CURRENCYCODE'] = urlencode('AUD');
$nvpreq = array_merge($nvpreq, $nvpArr);
$strReq = '';
foreach($nvpreq as $key => $val){
if($strReq!='') {
$strReq.= '&';
}
$strReq.= $key.'='.urlencode($val);
}
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $this->api_endpoint);
curl_setopt($ch, CURLOPT_VERBOSE, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $strReq);
$response = curl_exec($ch);
curl_close($ch);
print_r($this->deformatNVP($response));
}
added notes
Now i found out that it wasn't the problem with code, it was working with another username,password and signature in sandbox account though! So what might be the problem?