If you can’t add some products via Product Search page it usually means that you have an issue with Amazon Advertising API access keys or Tracking ID (Associate Tag). Regenerate API access keys in your Amazon Associate account and add a new pair of keys to corresponding fields at Ashop.co Settings page.
Another reason why the product can be declined by Product Search it’s a limits set by Amazon Advertising API.
Here is a quote from Amazon documentation:
Product Advertising API operations sometimes return items that cannot be added to the Active cart area. The following sections explain those cases:
Out of Stock Items
Most items for sale are available immediately. There are times, however, when that is not true. Items can go out of stock or very popular items, such as a new Harry Potter book, are pre-sold, that is, the book is sold before the book is even published.
When a customer adds an item to their cart that is not available, it is added to the SavedForLater area. Also, if an item in the cart, for some reason, becomes unavailable, Amazon automatically moves the item in the cart to the SavedForLater items section.
It is also possible for a customer to add an item directly to the SavedForLater items area in their cart so that they can easily purchase the item at a later date.
When items become available, you can use the CartModify operation with the Action parameter to move items from the SaveForLater section of the cart to the Active section, which is fully described in Moving Adding Items as Saved For Later.
Items With Limited Quantities
Many products have a limit on the quantity that can be added to a cart. Sometimes this is inherent to the product, for example, one-of-a-kind jewelry items. Sometimes it is a limit set by the vendor, for example, they might set a limit for big ticket items of one per customer. If you add an item to the cart with a quantity that exceeds the maximum,CartAdd or CartCreate automatically resets the quantity to the maximum. No error is generated. Therefore, you must verify the quantity of items in the cart with the number ordered to determine if the quantity has been capped. If so, you must notify the customer.
Digital Items, such as Kindle books, Amazon Instant Video, and MP3 files, can be purchased only by using Amazon 1-click. They cannot be placed in the cart. The following values for the Binding response element (ItemAttributes) are digital media formats:
Variation parent items cannot be added to a cart. For example, you cannot add Apparel to a cart because it is not clear what apparel you really want to buy. You could add one of its children, however, such as a red shirt, size large. If you try to add a parent item, you get an error message similar to the following.
The item you specified, [ASIN], is not eligible to be added to the cart. Check the item’s availability to make sure it is available.
Variation parents do not have corresponding OfferListingId's.
Collection parent items are abstractions of the items in the collection. The collection parent cannot be added to a cart. The Collection parent is used as a name holder for the collection. Collection parents do not have corresponding OfferListingId's.
Read full article on Amazon: Items That Cannot Be Added to the Active Cart Area