Access Denied When Rendering a Report Using System.Net.CredentailCache.DefaultCredentials

When I render a report on my local mahcine using the following code - it works correctly and the report is rendered - but with the same code - I'm getting an "401 - Access Denied" on another machine.  What would affet the "System.Net.CredentialCache.DefaultCredentials" so that it worked correctly in one instance but not in another?  

ReportingServices2005.ServerReportOutput output =  new .ReportingServices2005.ServerReportOutput();
 
string reporttype = Request.QueryString["Document"];
string reportname = "ReportName";
       
 output.ReportParameters.Add("ReviewID",Request.QueryString["ReviewID"]);
 output.ReportExecutionUrl = System.Configuration.ConfigurationManager.AppSettings["ReportServer"];
       
output.ReportExecutionCredentials = System.Net.CredentialCache.DefaultCredentials;
output.ReportPath = reportname;
output.OutputType = InfinityIncremented.ReportingServices2005.OutputType.AdobePDF;

thanks in advance for any help with this.