Hard to say without any supporting facts or examples to work with, but:
- Mechanics
One of the most basic parts of the game. Have you ever seen an enemy (or an ally) play one of your main champs and thought "Hmm, didn't know you could do that" or "Seems like he's doing pretty well!" Get a grasp on the limits of your champion. You don't have to be perfect to climb, just familiarize yourself with things like ability ranges and damage so you can pull off snap plays like Flash + Ability etc etc.
- CS
Farm up. CSing well can make up for getting behind in the game, even if you have no kills you can still get crucial items at the right times by farming consistently and well. It might be tempting to try to try to pad your performance with gung-ho plays, but being 0/0/0 and 30 CS up from your opponent is better than being 2/2/0 (since you're giving them money too). However don't let yourself be tunnel visioned by just farming, make sure you keep an eye on your team movements and push fast if a fight looks like it's brewing.
- Decision making/shotcalling
Ties in a little with the above. Even if you do well, there is still room for error. If you're fed but alone without teammates near you, you bet your soul that the enemy will consider piling on you with a numbers advantage. Make careful use of vision and shove the lane quickly for a back if you don't know where the enemy is. Additionally, you may want to play either more conservatively or with more risk depending on circumstance. Making a risky play has much room for failure, and you might fumble your abilities in the moments that count. So make sure you plan well ahead for what you're doing. On the flipside, if you're playing an assassin and feeling satisfied with just poking your opponents out of lane, this might not be the best strategy. Poke them down to a medium level instead and conserve your resources for an all-in and you might be able to push your advantage with a few kills.
Last bit here is communicating with your team. You don't have to always be a sheep when it comes to calls, don't be afraid to ping the team away from baron if the enemy team is alive and nearby, or tell them they don't need to towerdive if you're feeling skeptical. Apply this logic to your own play as well, don't set aside your doubts about what might go wrong and try to brute force a play, be more calculating.
One last tip is to write down what you did wrong each game, even if you think you played decently. If you're able to look back on your performance analytically, it might improve your future performance.
That's all I have offhand, hope it helps.
