There are only so many things that are able to decrypt from engrams. Particularly for chest armour. There are several armor pieces we know of that are in the armory but not in the game and a lot of weapons are from quests only. Make sure u know what all is able to decrypt, then if there is something particular u are after, u can keep trying. I've found that it helps to have all of the exotics u have of that type in your inventory when decrypting to avoid duplicates. This may just be me but I've seen some people backing it up. Also, leave some space between decrypting engrams of the same type. Rapidly doing them tends to get a lot of duplicates. I'm at the point now where I have almost everything that can decrypt and am holding onto any engrams I get for when I break 300.