Adding user to local administrators group via CMD does not seem to work

Question

I am trying to add a user to the local administrators group via my windows form app.

So in my Windows Form App I create the user in Active Directory (this part works great) then once the user is added the computer gets added to the domain (this part works great as well), then I need to add the user to the local administrator group on the computer (this is where it doesn't work)

So what I have is:

Process cmd = new Process();
        cmd.StartInfo.UseShellExecute = false;
        cmd.StartInfo.FileName = "cmd.exe";
        cmd.StartInfo.Arguments = "/C net localgroup administrators " + txt_LogonName.Text + " /add";
        cmd.StartInfo.Arguments = "pause";
        cmd.StartInfo.CreateNoWindow = false;
        cmd.StartInfo.RedirectStandardInput = true;
        cmd.Start();

the txt_LogonName.Text is where I specify the user details. IE domain\user

So normally running this command from CMD as admin

net localgroup administrators domain\user /add

works great, but does not want to run in my windows app.

Any ideas why?

Thanks