Why does setting the position of an external application only work correctly with breakpoint?

[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

[DllImport("user32.dll", SetLastError = true)]
static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

const uint SWP_NOSIZE = 0x0001;

public void SetWindowPosition(string txtAppTitle, string txtWidth, string txtHeight, string txtX, string txtY)
{
    IntPtr hWnd = FindWindow(null, txtAppTitle);

    // If found, position it.
    if (hWnd != IntPtr.Zero)
    {
        // Move the window to (txtX, txtY) without changing its size.
        int x = int.Parse(txtX);
        int y = int.Parse(txtY);
        SetWindowPos(hWnd, IntPtr.Zero, x, y, 0, 0, SWP_NOSIZE);
    }
}

private void btnOnScreenKeyboard_Click(object sender, EventArgs e)
{
    SetWindowPosition("On-Screen Keyboard", "0", "0", "155", "545");
}