morphToMany and morphedByMany

I have three tables Products ,Categories and SubCategories and I want to use morphToMany relation with coupons ex :

products : id name

Categories : id name

subCategories : id name

coupon id name

specification_coupon : id coupon_id specified_id specified_type

class Coupon extends Model {

public function ProductsCoupon()
{
    return $this->morphedByMany('App\Product', 'specified','specification_coupons');
}


public function CategoryCoupon()
{
    return $this->morphedByMany('App\Category', 'specified','specification_coupons');
}


public function SubCategoryCoupon()
{
    return $this->morphedByMany('App\SubCategory', 'specified','specification_coupons');
} 

}

in category , product and subcategory Models I do like this class Category extends Model { public function SpecifiedTypes(){ return $this->morphToMany('App\Coupon', ' specified');

}

}

but the relation doesn't work when I do

dd($coupon->ProductsCoupon)

I got this

Collection {#2195 ▼ #items: [] }

please tell me what I did wrong , my project is on laravel 5.7