ObservableCollection CollectionChanged doesn't help in WPF MVVM

I am using DataGrid and communicating using ObservableCollection in ViewModel

private ObservableCollection<StockItem> _stockList;
public ObservableCollection<StockItem> StockList
{
    get
    {
        return _stockList;
    }
    set
    {
        _stockList = value;
        OnPropertyChanged("StockList");                
    }
}

The StockItem class contains its Properties, which are a column in the DataGrid. The DataGrid has a column called Amount, whose values ​​are changed using the Quantitative Price column of the same data table.

I have a property called TotalAmount in the ViewModel that evaluates in the ObservableCollection CollectionChanged event, e.g.

void OnStockListChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
    this.TotalAmount = this.StockList.Sum(t => t.Amount);
}

These values ​​are only updated in the binding of the TextBox to TotalAmount when a new row is added to the DataGrid with some data. I want this TextBox TotalAmount to be updated as soon as the total column in the datagrid changes.

How can i do this.

StockItem Class

public class StockItem : ObservableObject, ISequencedObject
{
    JIMSEntities dbContext = new JIMSEntities();
    public StockItem()
    {
        var qs = dbContext.Stocks.Select(s => s.StockName);
        _stocks = new CollectionView(qs.ToArray());
        _stocks.CurrentChanged += new EventHandler(Stocks_CurrentChanged);
    }
    void Stocks_CurrentChanged(object sender, EventArgs e)
    {
        if (_stocks.CurrentItem != null)
            StockName = _stocks.CurrentItem.ToString();
        var qs = (from p in dbContext.Stocks
                  where p.StockName.Contains(StockName)
                  select new { Unit = p.Unit, UnitPrice = p.UnitPrice }).SingleOrDefault();
        if (qs != null)
        {
            Unit = qs.Unit;
            UnitPrice = (decimal)qs.UnitPrice;
        }
    }

    private CollectionView _stocks;
    public CollectionView Stocks
    {
        get
        {
            return _stocks;
        }
        set
        {
            _stocks = value;
            OnPropertyChanged("Stocks");
        }
    }
    private int _sNo;
    public int SNo
    {
        get
        {
            return _sNo;
        }
        set
        {
            _sNo = value;
            OnPropertyChanged("SNo");

        }
    }
    private string _stockName;
    public string StockName
    {
        get
        {
            return _stockName;
        }
        set
        {
            _stockName = value;
            OnPropertyChanged("StockName");
        }
    }
    private decimal _unitPrice;
    public decimal UnitPrice
    {
        get
        {
            return _unitPrice;
        }
        set
        {
            _unitPrice = value;
            OnPropertyChanged("UnitPrice");
            OnPropertyChanged("Amount");
        }
    }
    private string _unit;
    public string Unit
    {
        get
        {
            return _unit;
        }
        set
        {
            _unit = value;
            OnPropertyChanged("Unit");
        }
    }

    private decimal _discount;
    public decimal Discount
    {
        get
        {
            return _discount;
        }
        set
        {
            _discount = value;
            OnPropertyChanged("Discount");
            OnPropertyChanged("Amount");
        }
    }
    private decimal _quantity;
    public decimal Quantity
    {
        get
        {
            return _quantity;

        }
        set
        {
            _quantity = value;
            OnPropertyChanged("Quantity");
            OnPropertyChanged("Amount");
        }
    }
    public decimal Amount
    {
        get
        {
            decimal total = Quantity * (UnitPrice - (UnitPrice * (Discount / 100)));
            return total;
        }
    }        

    public override string ToString()
    {
        return StockName;
    }
}