x509 Certificate only working with one logon account

I have an application that is generating the following error:

System.ComponentModel.Win32Exception: Cannot find certificate and private key for decryption
at Microsoft.Web.Services2.Security.Cryptography.RSACryptoServiceProvider.AcquirePrivateKey(IntPtr certificateContext)
at Microsoft.Web.Services2.Security.Cryptography.RSACryptoServiceProvider..ctor(IntPtr certificateContext, X509KeyContainerContents contents)
at Microsoft.Web.Services2.Security.X509.X509Certificate.get_Key()
at WebProxy.Form1.GetSecurityToken() in C:\Inetpub\wwwroot\WebProxy1\WebProxy1
Form1.cs:line 518
at WebProxy.Form1.postFiles(String indentifier2) in c:\Inetpub\wwwroot\WebProxy1\WebProxy1\Form1.cs:line 132

Here are the relevant lines
132
X509SecurityToken signatureToken = GetSecurityToken();
 public X509SecurityToken GetSecurityToken()
        //This routine signs the file with the Aircheck Certificate
        {
            X509SecurityToken securityToken = null;
            //X509CertificateStore store = X509CertificateStore.CurrentUserStore(X509CertificateStore.MyStore);
            X509CertificateStore store = X509CertificateStore.LocalMachineStore(X509CertificateStore.MyStore);
            bool open = store.OpenRead();
            try
            {

                X509CertificateCollection certs = store.FindCertificateBySubjectString("sgoerdel@nctcog.org");

                Microsoft.Web.Services2.Security.X509.X509Certificate cert = ((Microsoft.Web.Services2.Security.X509.X509Certificate)certs[0]);
                if (cert == null)
                {
                    MessageBox.Show(
                        "You chose not to select an X.509 " +
                        "certificate for signing your messages.");
                    securityToken = null;
                }
   LINE 518             else if (!cert.SupportsDigitalSignature ||
            (cert.Key == null))
                {
                    MessageBox.Show(
                        "The certificate must support digital " +
                        "signatures and have a private key available.");
                    securityToken = null;
                }
                else
                {
                    securityToken = new X509SecurityToken(cert);
                }
            }
            finally
            {
                if (store != null)
                    store.Close();
            }
            return securityToken;
        }

This application is stored on our web server and one must log in to the server to access it.
This works absolutely fine when I log in. The certificates are located in the LocalMachineStore.
If anyone but me logs in, even other sytem administrators, it generates the error above.

X509SecurityToken signatureToken = GetSecurityToken();
 
public X509SecurityToken GetSecurityToken()
        //This routine signs the file with the Aircheck Certificate
        {
            X509SecurityToken securityToken = null;
            //X509CertificateStore store = X509CertificateStore.CurrentUserStore(X509CertificateStore.MyStore);
            X509CertificateStore store = X509CertificateStore.LocalMachineStore(X509CertificateStore.MyStore);
            bool open = store.OpenRead();
            try
            {
 
                X509CertificateCollection certs = store.FindCertificateBySubjectString("sgoerdel@nctcog.org");
 
                Microsoft.Web.Services2.Security.X509.X509Certificate cert = ((Microsoft.Web.Services2.Security.X509.X509Certificate)certs[0]);
                if (cert == null)
                {
                    MessageBox.Show(
                        "You chose not to select an X.509 " +
                        "certificate for signing your messages.");
                    securityToken = null;
                }
   LINE 518             else if (!cert.SupportsDigitalSignature ||
            (cert.Key == null))
                {
                    MessageBox.Show(
                        "The certificate must support digital " +
                        "signatures and have a private key available.");
                    securityToken = null;
                }
                else
                {
                    securityToken = new X509SecurityToken(cert);
                }
            }
            finally
            {
                if (store != null)
                    store.Close();
            }
            return securityToken;
        }

                                  

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