Why am I unable to scroll with the thumbtrack in this my scrolling program?

/// <summary>
/// Handle the <c>WM_VSCROLL</c> and <c>WM_HSCROLL</c> messages. Adjust the scroll information
/// block to re-display the application's view in an adjusted
/// vertical, or horizontal, position according to user selection.
/// Called in response to UP, DOWN, LEFT and RIGHT key or mouse commands
/// </summary>
///
/// <param name="hWnd">handle of the window</param>
/// <param name="wParam">scroll code and position value</param>
/// <param name="sbType">indicates vertical or horizontal movement</param>
///
/// <returns>No return value</returns>
///
void OnScrollMsg(
	HWND		hWnd,
	WPARAM		wParam,
	int			sbType
)
{
	UINT		uCode = LOWORD(wParam);
	int			nPosition = HIWORD(wParam); // if uCode is SB_THUMBPOSITION or SB_THUMBTRACK
	SCROLLINFO	scrollInfo;
	int			nOldPos;

	// get the current scroll position data and adjust as necessary
	ZeroMemory(&scrollInfo, sizeof scrollInfo);
	scrollInfo.cbSize = sizeof scrollInfo;
	scrollInfo.fMask = SIF_ALL;
	if (::GetScrollInfo(hWnd, sbType, &scrollInfo) == 0)
		scrollInfo.nPos = -1;	// first time
	nOldPos = scrollInfo.nPos;
	switch (uCode)
	{
	case SB_LINEUP:		// Scrolls up or
		//	case SB_LINELEFT:	// left by one unit.
		scrollInfo.nPos -= nPosition == 0 ? 1 : nPosition;
		break;
	case SB_LINEDOWN:	// Scrolls down or
		//	case SB_LINERIGHT:	// right by one unit.
		scrollInfo.nPos += nPosition == 0 ? 1 : nPosition;
		break;
	case SB_PAGEUP:		// Scrolls up by the height or
		//	case SB_PAGELEFT:	// left by the width of the window.
		scrollInfo.nPos -= scrollInfo.nPage;
		break;
	case SB_PAGEDOWN:	// Scrolls down by the height or
		//	case SB_PAGERIGHT:	// right by the width of the window.
		scrollInfo.nPos += scrollInfo.nPage;
		break;
	case SB_THUMBPOSITION:
		// actual position is stored in the info structure
		//scrollInfo.nPos = scrollInfo.nTrackPos;
		//break;
	case SB_THUMBTRACK:
		// actual position is stored in the info structure
		scrollInfo.nPos = nPosition;
		break;
	case SB_TOP:		// Scrolls to the top
		//	case SB_LEFT:		// or left of the window.
		scrollInfo.nPos = 0;
		break;
	case SB_BOTTOM:		// Scrolls to the bottom
		//	case SB_RIGHT:		// or right of the window.
		scrollInfo.nPos = scrollInfo.nMax + 1;
		break;
	default:
		break;
	}
	// ensure the position is not out of range
	if (scrollInfo.nPos < 0)
		scrollInfo.nPos = 0;
	if (scrollInfo.nPos >= scrollInfo.nMax - (int)scrollInfo.nPage)
		scrollInfo.nPos = scrollInfo.nMax - scrollInfo.nPage;

	if (nOldPos != scrollInfo.nPos)
	{
		// set the new position and force repainting of the screen
		::SetScrollInfo(hWnd, sbType, &scrollInfo, TRUE);
		::InvalidateRect(hWnd, nullptr, TRUE);
	}
}

/// <summary>
/// Set ranges for the scrollbar - vertical or horizontal.
/// Called from WM_PAINT handler for vertical and horizontal scrolling
/// </summary>
///
/// <param name="nBar"><c>SB_xx</c> value indicating whether vertical or horizontal bar</param>
/// <param name="nPageSize">height or width of a visual page</param>
/// <param name="nMaxRange">height or width of total data to view</param>
///
/// <returns>Current position of the scroll box</returns>
///
int SetScrollRange(
	HWND		hWnd,
	int			nBar,
	int			nPageSize,
	int			nMaxRange
)
{
	SCROLLINFO	scrollInfo;

	// Add/adjust vertical scroll bars...
	ZeroMemory(&scrollInfo, sizeof scrollInfo);
	scrollInfo.cbSize = sizeof scrollInfo;
	scrollInfo.fMask = SIF_ALL;

	// get the current information
	::GetScrollInfo(hWnd, nBar, &scrollInfo);

	// set the new values
	scrollInfo.nPage = nPageSize;	// lines/characters in the page
	scrollInfo.nMax = nMaxRange;	// total number of lines/characters
	::SetScrollInfo(hWnd, nBar, &scrollInfo, TRUE);

	return scrollInfo.nPos;
}

// Use the view and character sizes to set the scrolling sizes
int viewHeight = paintStruct.rcPaint.bottom - paintStruct.rcPaint.top;
// Set the starting line of dats to be painted
int nLine = SetScrollRange(hWnd, SB_VERT, viewHeight / static_cast<int>(charHeight), pVpstr->size());

int viewWidth = paintStruct.rcPaint.right - paintStruct.rcPaint.left;
// set the rightmost character position: maxWidth is the width of the widest line to paint
int nChar = SetScrollRange(hWnd, SB_HORZ, viewWidth / static_cast<int>(charWidth), maxWidth);