perhaps he doesn't care for the food very much. And to be honest, Pedigree is not a very good food for your pup. I'm not exactly what foods you have available in the UK - but here is a link to some foods & you could compare the different foods & ingredients. Educating yourself about food is important. Many choose to homecook. Here's the link
www.dogfoodanalysis.com -- most yorkie owners avoid 6 star foods because they are very high in protein, good choices may be the 4 or 5 star foods