PowerPoint Launched via C# does not Quit

Hey I'm automating PowerPoint and Excel from a C# WinForms application; what I do is read slides from PowerPoint and save them in Excel and then quit both apps. Excel quits successfully but PowerPoints doesn't quits. The problem is when I convert first time it doesnt quits, but when I convert again it does.

Here is my code

try
{
    PowerPoint.Application ppApp;
    PowerPoint.Presentation ppPres;
    List<Company> companies = new List<Company>();

    ppApp = new PowerPoint.Application();
    ppApp.Visible = Microsoft.Office.Core.MsoTriState.msoTrue;
    ppApp.WindowState = Microsoft.Office.Interop.PowerPoint.PpWindowState.ppWindowMinimized;

    ppPres = ppApp.Presentations.Open(fileTxtBox.Text,
                                      Microsoft.Office.Core.MsoTriState.msoFalse,
                                      Microsoft.Office.Core.MsoTriState.msoFalse,
                                      Microsoft.Office.Core.MsoTriState.msoTrue);

    int slides = ppPres.Slides.Count;

    for (int slide = 1; slide <= slides; slide++)
    {
        int rows = 1;
        PowerPoint.Cell cell;
        int shape = 1;

        for (; shape < ppPres.Slides[slide].Shapes.Count; shape++)
        {
            if (ppPres.Slides[slide].Shapes[shape].HasTable == Microsoft.Office.Core.MsoTriState.msoTrue)
            {
                cell = ppPres.Slides[slide].Shapes[shape].Table.Cell(1, 1);

                if (cell.Shape.TextFrame.TextRange.Text.Trim().ToLower().Contains("realized"))
                {
                    rows = ppPres.Slides[slide].Shapes[shape].Table.Rows.Count;
                    break;
                }
            }
        }

        Company comp = new Company(rows);
        InitializeCompany(ref comp, ppPres.Slides[slide]);
        companies.Add(comp);
    }

    SaveInExcel(companies);

    ppPres.Close();
    ppPres = null;
    ppApp.Quit();
    ppApp = null;

    return;
}

catch (Exception ex)
{
    MessageBo