C# WEB: Strange file download problem

[aepheus]

I use the following code to create a file for the user to download:
Response.Clear();
Response.ClearContent();
Response.ClearHeaders();
Response.Buffer = true;
Response.ContentType = "text/ris";
Response.AddHeader("Content-Disposition", "attachment;filename=" + sUserID + "-" + DateTime.Now.ToFileTime() + ".ris");
Response.Charset = "UTF-8";
Response.ContentEncoding = System.Text.Encoding.UTF8;
Response.Write(sFormatted);
Response.Flush();
Response.End();

I generate sFormatted in 2 different ways depending on how the user creates the file.

In the first type of file generation, the user provides a comma delimited list of IDs (through a hidden form element) from which a string is created of the articles corresponding to the IDs.

In the second type, the user provides a Group ID (through a hidden form element) from which a set of IDs are obtained from an Object corresponding to that Group ID. A string is then created of the articles corresponding the the set of IDs.

If I pass a set of IDs identical to the set returned by a Group ID, I get an identical set of IDs from both methods, I get an identical string from both methods but only the first method prompts for a download, the second method just reloads the page (a postback is performed to get the hidden element to the server). No exceptions are thrown during or after the generation of the string and both methods go through the same code after the string is generated.

Any ideas on what might be causing the second method to not prompt the download? Session Objects are being called in the second method.