You only need four researched traits for the item you want to craft. If you want to craft a Lightning Staff you need four researched traits for Lightning Staffs. That said, try researching everything. Researching takes a lot of time, especially the last few traits (it takes over 30 real life days to research the last, ninth trait). There are a few sets requiring eight or nine researched traits. Regarding your problem. Make sure you are at the proper location. You can only craft them at certain locations, as you may know. You can simply Google that location. Once you're at the proper location and crafting station, make sure you select set crafting (with LB/RB on Xbox). Normally the standard crafting is selected, you have to switch that to craft items of the set. If you done it correctly you will also notice the name of the item changes. For example, a Lightning Staff usually is just named "Ruby Ash Lightning Staff" (if you select the highest material grade, Sanded Ruby Ash). If you switch to the set, it would say in your case "Magnus's Lightning Staff".