Each technology you research has a Tier Level associated with it. You must research a Tier 1 technology before researching a Tier 2, and so on and so forth. I believe you can check where you stand by going through your research tree and looking at each tech you have researched. Look for the highest ones and see which Tier they fall under. Just be advised, Tier 6 takes 192 Hours to research, so it basically takes about 1 month or more get to that level, depending on your research pattern.