It works for me. Could be something wrong in your signature. AssociateTag is your 'Associates ID' that you get after sign up for affiliate not any tracking ID. And I sign for Amazon.co.uk Associates.
<ItemLookupResponse xmlns="http://webservices.amazon.com/AWSECommerceService/2011-08-01">
<OperationRequest>
<HTTPHeaders>
<Header Name="UserAgent" Value="Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.146 Safari/537.36"/>
</HTTPHeaders>
<RequestId>5f5e44b1-8005-4b27-846a-9959775bdf24</RequestId>
<Arguments>
<Argument Name="Operation" Value="ItemLookup"/>
<Argument Name="Service" Value="AWSECommerceService"/>
<Argument Name="Signature" Value="Vfto8GdjP875KGwYrpe1QyfDkpUr3HDkQAyAcyyI4n0="/>
<Argument Name="AssociateTag" Value="jay048-21"/>
<Argument Name="Version" Value="2009-03-31"/>
<Argument Name="ItemId" Value="0545010225"/>
<Argument Name="AWSAccessKeyId" Value="AKIAIAAPUKLVP6OGAT4Q"/>
<Argument Name="Timestamp" Value="2014-03-05T17:14:31Z"/>
<Argument Name="ResponseGroup" Value="Small"/>
</Arguments>
<RequestProcessingTime>0.0143640000000000</RequestProcessingTime>
</OperationRequest>
<Items>
<Request>
<IsValid>True</IsValid>
<ItemLookupRequest>
<IdType>ASIN</IdType>
<ItemId>0545010225</ItemId>
<ResponseGroup>Small</ResponseGroup>
<VariationPage>All</VariationPage>
</ItemLookupRequest>
</Request>
<Item>
<ASIN>0545010225</ASIN>
<DetailPageURL>
http://www.amazon.com/Harry-Potter-Deathly-Hallows-Book/dp/0545010225%3FSubscriptionId%3DAKIAIAAPUKLVP6OGAT4Q%26tag%3Djay048-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D165953%26creativeASIN%3D0545010225
</DetailPageURL>
<ItemLinks>
<ItemLink>
<Description>Technical Details</Description>
<URL>
http://www.amazon.com/Harry-Potter-Deathly-Hallows-Book/dp/tech-data/0545010225%3FSubscriptionId%3DAKIAIAAPUKLVP6OGAT4Q%26tag%3Djay048-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D386001%26creativeASIN%3D0545010225
</URL>
</ItemLink>
<ItemLink>
<Description>Add To Baby Registry</Description>
<URL>
http://www.amazon.com/gp/registry/baby/add-item.html%3Fasin.0%3D0545010225%26SubscriptionId%3DAKIAIAAPUKLVP6OGAT4Q%26tag%3Djay048-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D386001%26creativeASIN%3D0545010225
</URL>
</ItemLink>
<ItemLink>
<Description>Add To Wedding Registry</Description>
<URL>
http://www.amazon.com/gp/registry/wedding/add-item.html%3Fasin.0%3D0545010225%26SubscriptionId%3DAKIAIAAPUKLVP6OGAT4Q%26tag%3Djay048-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D386001%26creativeASIN%3D0545010225
</URL>
</ItemLink>
<ItemLink>
<Description>Add To Wishlist</Description>
<URL>
http://www.amazon.com/gp/registry/wishlist/add-item.html%3Fasin.0%3D0545010225%26SubscriptionId%3DAKIAIAAPUKLVP6OGAT4Q%26tag%3Djay048-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D386001%26creativeASIN%3D0545010225
</URL>
</ItemLink>
<ItemLink>
<Description>Tell A Friend</Description>
<URL>
http://www.amazon.com/gp/pdp/taf/0545010225%3FSubscriptionId%3DAKIAIAAPUKLVP6OGAT4Q%26tag%3Djay048-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D386001%26creativeASIN%3D0545010225
</URL>
</ItemLink>
<ItemLink>
<Description>All Customer Reviews</Description>
<URL>
http://www.amazon.com/review/product/0545010225%3FSubscriptionId%3DAKIAIAAPUKLVP6OGAT4Q%26tag%3Djay048-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D386001%26creativeASIN%3D0545010225
</URL>
</ItemLink>
<ItemLink>
<Description>All Offers</Description>
<URL>
http://www.amazon.com/gp/offer-listing/0545010225%3FSubscriptionId%3DAKIAIAAPUKLVP6OGAT4Q%26tag%3Djay048-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D386001%26creativeASIN%3D0545010225
</URL>
</ItemLink>
</ItemLinks>
<ItemAttributes>
<Author>J. K. Rowling</Author>
<Creator Role="Illustrator">Mary GrandPré</Creator>
<Manufacturer>Arthur A. Levine Books</Manufacturer>
<ProductGroup>Book</ProductGroup>
<Title>Harry Potter and the Deathly Hallows (Book 7)</Title>
</ItemAttributes>
</Item>
</Items>
</ItemLookupResponse>