Make sure that the sixth trait isn't showing as "researching." Assuming you're trying to craft a set like Julianos that requires six traits, you don't get credit for the trait until you've completely finished researching it.

Another possibility - You can only craft items in the traits you've actually researched. Just because you have the six traits unlocked to be able to craft that item, you still need the trait you're trying to craft to be one of the ones previously researched.

If you meet the above requirements already, then it could possibly be a bug, or you don't have the appropriate trait stone in your inventory.

Just double check to make sure one of the above listed situations is not occurring.

Also, you need to have the correct number of traits learned on the item you want to craft. So, if you're wanting to craft a robe, you have to have researched the correct number of traits for robes. Even if you've researched 6 traits for breeches, you can't craft a robe in a 6-trait set unless you've researched 6 traits for robes.