You can only craft a given selection of "prefixes". Carrion will always be Power/Vit/CondDmg, Knights will always be Power/Tough/Precision, etc. So if the 'combo' you want isn't one of the prefixes, it's not craftable. The X factor being a lot/some of the dungeon sets and Karma sets (purchasable at the Orrian temples) have non-craftable stat combos. For instance, "Soldiers" gear is not craftable but you are still able to purchase Power/Vit/Toughness gear from dungeons and Orrian karma vendors. If you can't find what you want from crafting, browse those.