WH_KEYBOARD_LL not working for regular letters and digits

Question

I'm using SetWindowsHookExA(WH_KEYBOARD_LL, HookCallback, GetModuleHandleA(NULL), 0); to set a global hook for capturing the keystrokes, but the result is strange.
The callback function can be executed when I press the "special" keys such as "Enter", "Tab", "Shift", "Ctrl" and other keys having a Virtual Key Code, while it fails to capture the keystrokes when I press the regular letters and digits.
I am confused about it and could anyone tell me the reason?

#include <Windows.h>
#include <iostream>
using namespace std;

HHOOK keyboardHook = 0;

LRESULT CALLBACK HookCallback(int code, WPARAM wParam, LPARAM lParam)
{
    KBDLLHOOKSTRUCT *ks = (KBDLLHOOKSTRUCT*)lParam;
    cout<< "[TEST] " << ks->vkCode << endl;

    return CallNextHookEx(0, code, wParam, lParam);
}

int main()
{
    keyboardHook = SetWindowsHookExA(WH_KEYBOARD_LL, HookCallback, GetModuleHandleA(NULL), 0);
    if (keyboardHook == 0)
    {
        cout << "failed" << endl;
        return -1;
    }
    cout << "ok" << endl;

    MSG msg;
    while(GetMessage(&msg, NULL, 0, 0)){
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    UnhookWindowsHookEx(keyboardHook);


    return 0;
}