Permission to perform the operation is denied

[Krishna-bizbazar]

Hi,
I am trying to use webservice for Contact update but could not update the contact. I have gone through Update.php webservice and found that webserviceObject empty. $webserviceObject = VtigerWebserviceObject::fromId($adb,$idList[0]);
The function 'fromId' of class VtigerWebserviceObject returns empty object though I can get the values of $rowData from function 'fromId'.
The version I am using is 7.5.
Could you please help me resolve the issue?