problems to add data in pivot table laravel 5.1

I hope you are fine. I have a problem with a pivot table in laravel 5.1 I want it that several products are inserted into this table , which have made ​​relations with two more tables . The problem is that inserts well ids products, but the extra fields such as quantity and price would not add them all , that is , I only adds a value of one product, do not know if I explained well . Of all forms attached photos of models , drivers and user view with the database . Is there something missing I do? Please if you can help me . Beforehand thank you very much!! PD: Sorry my english. I´m from latin america and I don´t speak english very well.

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Bill model

class Bill extends Model

{

protected $table = 'bills'; protected $fillable = ['date_at'];

public function products()
{
    return $this->belongsToMany('App\Product','bill_product','product_id', 'bill_id','dquantity','dprice')->withTimestamps();
}

}

Product Model

class Product extends Model

{

protected $table = 'products';
protected $fillable = ['name', 'price','stock'];

public function bills() { return $this->belongsToMany('App\Bill','bill_product','bill_id', 'product_id','dquantity','dprice')->withTimestamps(); } }

Controller

 public function store(Request $request)
{

    $fac = Bill::create($request->all());
    //Obtener ids de lineas de productos
    $productIds = $request->input('product_id');
    $cant= $request->input('dquantity');
    $prec=$request->input('dprice');


        $fac->products()->attach($productIds , ['dprice' => $prec,'dquantity' => $cant]);

}

View

</td>   
                                    <td>
                                        {!!Form::select('product_id[]',$produ,null,['class'=>'form-control'])!!}    
                                    </td>                                   

                                    <td width="20%">


                                        {!!Form::text('dquantity', null,['class'=>'form-control','id'=>'cantidad'])!!}

                                    </td>

via Chebli Mohamed