Render time only equal half of fps

I'm working on a project that remakes mario 3 (NES). In my game loop, i face a strange thing. Here is my code:

 while (true) { if (PeekMessage(&msg, 0, 0, 0, PM_REMOVE)) { if (msg.message == WM_QUIT) { break; } TranslateMessage(&msg); DispatchMessage(&msg); } else { //Game logic c1++; _Mario.Update(1.0 / FPS); //Render _Timer.SetBeginTick(); _LpD3dDevice9->Clear(0, 0, D3DCLEAR_TARGET, D3DCOLOR_XRGB(102, 255, 255), 1.0f, 0); if (_LpD3dDevice9->BeginScene()) { c2++; _Mario.Render(); _LpD3dDevice9->EndScene(); } _Keyboard->GetState(); _Mario.Move(_Keyboard); _LpD3dDevice9->Present(0, 0, 0, 0); _Timer.SetCurrTick(); _Timer.UpdateGameTIme(); if (_Timer.ElapsedTime < (1.0 / FPS)) { Sleep(1000.0 / FPS - _Timer.ElapsedTime * 1000); } } } 

Update function :

void ImageMove::Update(float t) { _Image_X += _Image_Vx * t; _Image_Y -= _Image_Vy * t; _SpriteTimer.SetCurrTick(); _SpriteTimer.UpdateGameTIme(); if (_SpriteTimer.ElapsedTime >= 1.0 / ANIMATE_RATE) { if (_Image_Vx > 0) _Image_Right.Next(); //change to next sprite if (_Image_Vx < 0) _Image_Left.Next(); // change to next sprite _SpriteTimer.SetBeginTick(); } } 

Render Function :

void ImageMove::Render() { int vpx = _Image_X - 400; int VIEW_PORT_Y = _Image_Y + 500; _SpriteHandler->Begin(D3DXSPRITE_ALPHABLEND); if (_Image_Vy > 0 && _Image_Vx_Last > 0) _Image_Jump_Right.Render(_Image_X, _Image_Y); else if (_Image_Vy > 0 && _Image_Vx_Last < 0) _Image_Jump_Left.Render(_Image_X, _Image_Y); else if (_Image_Vx > 0)_Image_Right.Render(_Image_X, _Image_Y); else if (_Image_Vx < 0)_Image_Left.Render(_Image_X, _Image_Y); else if (_Image_Vx_Last < 0)_Image_Left.Render(_Image_X, _Image_Y); else _Image_Right.Render(_Image_X, _Image_Y); _SpriteHandler->End(); } 

I put c1 and c2 in my game loop to measure how many times Update and Render are called. I set fps = 60, while debuging, I let my game loop run for 5s, then set a breakpoint to view c1 and c2. The value of c1 = 320, and c2 = 160 ( = 1/2 c1 for sure, i have run it many times, so the times render is called only ~1/2 fps, i think with fps = 60, it should be ~300 for 5s, shoudn't It?).

If i debug with F10 (on Visual studio) , I can see a loop will execute with _LpD3dDevice9->BeginScene() success, and _LpD3dDevice9->BeginScene() in next loop always failed.

Is it error? Because i have tested on another small project, and c2 = 1/2 c1 too. This problem cause an issue in my game. In Update() function, I change my sprite here, and draw it in Render() , so I always miss some sprite. Because It only render = 1/2 time game update.

If it is an error How can I fix my game loop? And if It is not an error, what can I do to prevent missing sprites?

P/S : sorry for bad English.