XML Feed for Google Merchant Centre

I am trying to create an XML file, so our Google Merchant Centre products are automatically updated, but I can't get 'brand' or 'link' to populate. Can anyone help me identify the problem with my macros? Here's my code (I placed ** around the problem lines):@item@ @g:id@{% SKUName %}@/g:id@ @title@{% SKUName %}@/title@ @description@{% SKUShortDescription %}@/description@ **@link@{% GetSKUURL %}@/link@** @g:price@{% SKUPrice %} GBP@/g:price@ @g:availability@in stock@/g:availability@ @g:image_link@http://185.26.149.74/webimages/{% SKUName %}@/g:image_link@ **@g:brand@{% GetManufacturer %}@/g:brand@** @g:mpn@{% SKUName %}@/g:mpn@