Amazon itemLookup not returning new and used count

Question

I'm hitting the Amazon Product API with this function:

public function itemLookup($id, $responseGroup = 'OfferFull,Images,BrowseNodes,ItemAttributes,Offers'){

    $parameters = array(
    "IdType" => "ASIN",
    "Operation" => "ItemLookup",
    "ItemId" => $id,
    "ResponseGroup" => $responseGroup,
    "Condition" => "Used"
    );

    $signedUrl = $this->getSignedUrl($parameters);

    //return XML to front end
    return $productId = simplexml_load_file($signedUrl);
 }

It works and returns the XML document.

However, it doesn't return anything in the:

<OfferSummary>
    <TotalNew>0</TotalNew>
    <TotalUsed>0</TotalUsed>
    <TotalCollectible>0</TotalCollectible>
    <TotalRefurbished>0</TotalRefurbished>
</OfferSummary>
<Offers>
    <TotalOffers>0</TotalOffers>
    <TotalOfferPages>0</TotalOfferPages>
    <MoreOffersUrl>0</MoreOffersUrl>
</Offers>

In this particular example, I can visit the product URL and see there are new and used products available.

It seems the API chokes when the item has multiple colors/sizes/options.

Anyone have any idea how I can get around this?