Using automapper.collection in aspnet core & ef core doesn't updated edited records in a collection

I'm using asnet boilerplaite aspnet core and ef core. I've initialized automapper.collection in PreInitialize as follows:

Configuration.Modules.AbpAutoMapper().Configurators.Add(
    cfg =>{
        cfg.AddCollectionMappers();
        cfg.CreateMap<OrderDTO, Order>().EqualityComparison((odto, o) => odto.ID == o.ID);
        cfg.CreateMap<OrderItemDTO, OrderItem>().EqualityComparison((odto, o) => odto.ID == o.ID);
    }
);

I'm using ObjectMapper.Map(odto, o); to map between the dto and entity, but what I find are new orderitem in the orderitems collection within orders save fine, but any edits to existing entries seem to not update to the db.

The order entity and collection are loaded from the repository using the following:

var @item = await _orderRepository.GetAll().Include(x => x.Items).Include(x => x.Items).ThenInclude(x => x.Product).AsNoTracking().FirstOrDefaultAsync(x => x.Id == id);

here is the Update method:

    public override async Task<OrderDto> Update(OrderDto input)
    {
        var ord = await _orderManager.GetA(input.Id);

        ObjectMapper.Map(input, ord);

        CheckErrors(await _orderManager.UpdateA(ord));

        return MapToEntityDto(await _orderManager.GetA(input.Id));
    }

The values of input and ord variables are as follows when trying to update the orderitem record:

    input   {OrderDto}  
    Code    "1" string
    Id  1   int
    Items   Count = 1   System.Collections.Generic.ICollection<OrderItemDto>
    -       [0] {OrderItemDto}  
                Id  1   int
                OrderId 1   int
        +       Product {ProductLoadDto}
                ProductId   4   int
                Quantity    22  double
    MovementDate    {28-Jul-18 2:36:41 AM}  System.DateTime


    ord {[Order 1]} Order
    Code    "1" string
    Id  1   int
    Items   Count = 1   System.Collections.Generic.ICollection<OrderItem>
    -       [0] {OrderItem 1}
            Id  1   int
            OrderId 1   int
    +       Product {[Product 4]}   Venues.Products.Product
            ProductId   4   int
            Quantity    1   double
    MovementDate    {28-Jul-18 12:06:41 PM} System.DateTime

Is there something I'm missing to get automapper.collection to handle the collection edits automatically ?