Laravel not adding data to foreign key column value in the database.

I'm not getting an error message from this, but Laravel 5.1 will not assign values to a foreign key column in the OrderDetails table. I've tried multiple different calls and can't figure out what's wrong.

    public function orderProduct(Request $request){
        //Try and find the current incomplete order. If find fails, then create new order.
        $order;
        try{
            $order = OrderHeader::where("UserID","=", Auth::user()->id)
            ->where('OrderCompleted', '=', false)->firstOrFail();
        }catch(\Illuminate\Database\Eloquent\ModelNotFoundException $e){
            $order = new OrderHeader;
            //find user
            $id = Auth::user()->id;
            $user = User::find($id);
            //associate user
            $order->user()->associate($user);
            //mark order incomplete
            $order->OrderCompleted = false;
            $order->OrdersShipped = false;
            $order->save();
        }
        //find matching or create new order detail. Update or add details and ade it to order. 
       /**
        *Somewhere in here is the problem(s)
        *The four or so lines I've commented out are all different attempts
        *to get the value in the database.
        */
        $orderDetail;
        $productID = $request->input("pID");
        try{
            $orderDetail = $order->orderDetails()->where("ProductID", "=", $productID)->firstOrFail(); 
            //add feilds assignment
        }catch(\Illuminate\Database\Eloquent\ModelNotFoundException $e){
            $orderDetail = new OrderDetail;
//  $orderDetail->orderHeader()->associate(OrderHeader::find($order->OrderID));
            $orderDetail->QtyOrdered = $request->input("qty");
            $orderDetail->product()->associate(Product::find($productID));
//          $orderDetail->OrderHeaderID = $order->OrderID;
//          $orderDetail->orderHeader()->associate($order);
        //  $orderDetail->save();
        //$order->orderDetails()->save($orderDetail);
            //$orderDetail = OrderDetail::create(['OrderHeaderID' => $order->OrderId,'ProductID'=> $request->input("pID"), 'QtyOrdered' => $request->input("qty")]);
        }
        return $orderDetail;    
//  ProductController::cartView($order);
    }

The relationship bewtween OrderHeader and OrderDetails are as follows.

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class OrderDetail extends Model
{
    //
    protected $table = "OrderDetails";
    protected $primaryKey = "OrderDetails";
    protected $fillable = array('OrderHeaderID, ProductID, QtyOrdered');

    public function orderHeader(){
        return $this->belongsTo('App\OrderHeader', 'OrderHeaderID', 'OrderID');
    }
    public function product(){
        return $this->belongsTo('App\Product', 'ProductID', 'pID');
    }
}

And:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\HasMany;

class OrderHeader extends Model
{
    //
    protected $table = 'OrderHeader';
    protected $primaryKey = 'OrderID';
    protected $fillable = array('ShippingID');  

    public function orderDetails(){
        return $this->hasMany('App\OrderDetail','OrderHeaderID','OrderID');
    }
    public function shippingAddress(){
        return $this->belongsTo("App\Shipping", 'ShippingID', 'ShippingID');
    }
    public function user(){
        return $this->belongsTo("App\User", 'UserID');
    }
}

via Chebli Mohamed