The problem is that your teammates sucks and that nyx is team dependent if he wants to win a game.
Early game (after 6):
It's better to find a partner to gank with (roaming mids) because Nyx synergises well with people who can gank with him and secure a kill. Once he's done with his burst ( around ~495 dmg with reduction for Ult + maxed Q+ 2 AA), it's hard for nyx to follow up and chase that's why you need to have a partner to either do enough dmg to secure a kill (extra 300-500 dmg) or bring some minor/major CCs to secure the kill.
So always have a partner with you when you gank. If you see someone's who's killable, don't start just yet. Call someone and set up a gank.
Nyx can set up a fight with his stealth, multiple target stuns and mana burn some casters to disable them. But to win the teamfight, Nyx's teammates needs to follow up and start doing dmg at the same time as nyx strikes. He doesn't have enough continual dps to sutain during a fight nor has enough DPS to solo a squish (unless he has dagon but that requires a lot of farm). As I said before, his role is to set up a kill for the assassin type heroes on your team.
Always make sure your Assassin/melee fighter can follow up and be bffs with him.
Also always type Nyx in All chat after a kill.