Asp.net VB Web Service - The Operations times out

[belltric]

I have a web service application project which need to deploy in production server(windows server 2008 r2 standard) and UAT server(web server). One of my web service in the web service application will need to call to a third party web service which I named it as Service A. I manage to connect to Service A in my UAT successfully but I failed to connect in production server which results in the operation times out. My web service application called the production server failed but I can browse the third party web service in the browser. Both are having the same configuration steps.

Anyone know what might cause this?

Configuration that are done
install .pfx
import .cert (personal & current user)
give permission (IUSR & ASP.NET for web server)

VB Code

Dim cert As New X509Certificate2(HttpContext.Current.ApplicationInstance.Server.MapPath("~/cert/") & ConfigurationManager.AppSettings("CERTIFICATE"), "1234")
ws.ClientCertificates.Add(cert)

Web.Config

<appSetting><add key="CERTIFICATE" value="cert.pfx"/>
</appSettings>

Production Error

An error occurred. Error:
System.Net.WebException: The operation has timed out
at System.Web.Services.Protocols.WebClientProtocol.GetWebResponse(WebRequest request)
at System.Web.Services.Protocols.HttpWebClientProtocol.GetWebResponse(WebRequest request)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)