As you gain more muscle, your gains will become slower over time unless you imply some big, positive changes. You may be at the point where you work just as hard, but the muscle doesn't come as easily. For example, take someone who has never worked out before and is generally lazy in their lifestyle. Put them on a strict diet with a great routine and they could easily gain 20 lbs or more of muscle in a year, without supplements.
Then take a pro bodybuilder who has been working for years. They are working harder than almost anyone else on the planet (bb wise) and have a perfect diet, yet they may only gain a few lbs a year.
Just keep working hard and the gains will come in time. As far as a new supplement to take, get some creatine if you haven't already. Personally, I stay away from NO products. They really only help with the pump (which has not been scientifically proven to be of benefit to muscle growth) and there are a good number of health concerns. You can continue to use it if you wish, but you could save some money and be fine without it, imo.
Don't even think about trying steroids for another few years (21 at the earliest, preferably later). Your body is likely still developing and shouldn't be influenced by AAS at this point.
Well, I hope I could be of at least a little help. I know there are plenty of other guys here more experienced than me in this field, so hopefully they drop in. | 
