Cant imagine whats the problem with this simple program

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MFC

Hi. Im a novice to MFC programming. And I was just writing some simple tic tac toe program. My codes are shown below. It only draws X in a square where "l button down" messages occur. But mysteriously it fails to draw Xs in all the squares. If I click in all the squares, it only draws Xs for like 3 or 4 times. After that it stops working even though I click on a square. No errors. No beep message beep sounds. It just stops. But I cant understand where the error is.

Heres my code in header file.

<blockquote class="quote"><div class="op">Quote:</div>

class MainWindow : public CFrameWnd
{
public:
	MainWindow()
	{
		Create(NULL, L"WINDOW");
		
	}




protected:
	afx_msg void OnPaint();

	afx_msg void OnLButtonDown(UINT, CPoint);

private:
	void DrawGrid(CDC*);

	void DrawO(CDC*, int);  //Ignore this

	void DrawX(CDC*, int);


	DECLARE_MESSAGE_MAP()
};



class App : public CWinApp
{
public:
	BOOL InitInstance()
	{
		m_pMainWnd = new MainWindow;
		m_pMainWnd->ShowWindow(m_nCmdShow);
		m_pMainWnd->UpdateWindow();

		CRect rec(0, 0, 300, 300);
		m_pMainWnd->CalcWindowRect(rec);
		m_pMainWnd->SetWindowPos(NULL, 0, 0, rec.Width(), rec.Height(), SWP_NOZORDER | SWP_NOMOVE | SWP_NOREDRAW);
		
		return TRUE;
	}
};</blockquote>

And here is the cpp file.

#include "stdafx.h"
#include "MFCApplication15.h"

#define O 1
#define X 2


CRect sqs[9] =
{
	{ 0, 0, 100, 100 },
	{ 100, 0, 200, 100 },
	{ 200, 0, 300, 100 },

	{ 0, 100, 100, 200 },
	{ 100, 100, 200, 200 },
	{ 200, 100, 300, 200 },

	{ 0, 200, 100, 300 },
	{ 100, 200, 200, 300 },
	{ 200, 200, 300, 300 }
};

int vals[9]{0, 0, 0, 0, 0, 0, 0, 0, 0};

void MainWindow::OnPaint()
{
	CClientDC dc(this);
	DrawGrid(&dc);

	for (int i{}; i < 9; i++)
		if (vals[i] == X)
			DrawX(&dc, i);
}

void MainWindow::OnLButtonDown(UINT flags, CPoint cp)
{
	for (int i{}; i < 9; i++)
	{
		if (PtInRect(sqs[i], cp))
			vals[i] = X;
	}
}

void MainWindow::DrawGrid(CDC* pDC)
{
	CPen *pen = new CPen(PS_SOLID, 5, RGB(0, 0, 0));
	pDC->SelectObject(pen);

	pDC->MoveTo(0, 100);
	pDC->LineTo(300, 100);
	pDC->MoveTo(0, 200);
	pDC->LineTo(300, 200);

	pDC->MoveTo(100, 0);
	pDC->LineTo(100, 300);
	pDC->MoveTo(200, 0);
	pDC->LineTo(200, 300);
}


void MainWindow::DrawO(CDC* pDC, int pos)  //Ignore this
{
	CPen* pen = new CPen(PS_SOLID, 3, RGB(255, 0, 0));

	pDC->SelectObject(pen);

	pDC->Ellipse(sqs[pos]);
}

void MainWindow::DrawX(CDC* pDC, int pos)
{
	CPen* pen = new CPen(PS_SOLID, 3, RGB(0, 0, 255));
	pDC->SelectObject(pen);

	pDC->MoveTo(sqs[pos].left, sqs[pos].top);
	pDC->LineTo(sqs[pos].right, sqs[pos].bottom);
	pDC->MoveTo(sqs[pos].right, sqs[pos].top);
	pDC->LineTo(sqs[pos].left, sqs[pos].bottom);
}

App myApp;

BEGIN_MESSAGE_MAP(MainWindow, CFrameWnd)
	ON_WM_PAINT()
	ON_WM_LBUTTONDOWN()
END_MESSAGE_MAP()

What's the wrong with my code ? Why it stops the drawing after like 4 clicks ?
Thanks in advance.