Hey there. I main LB in mid, and I used to have that same problem. Her kit dominates the early game, but the longer the game foes the harder it is to win. Her kit early plays itself to a degree. Q W Thunderlords and there goes most their HP, but late game diving in is unsafe due to her frail nature and lack of resistance. All she has is the invis and distortion pad. However cc ruins has, as it blocks distortion damage.
To win with LeBlanc, you want to snowball. Always go Bounty Hunter in masteries, because the damage ramps up very quickly. Try to kill the enemy laner, then roam. Roaming is essential to her success. Her early damage and mobility are insane. Ethereal Chains cc will help your laner catch up, and the combination of damage will surely be enough to bring them down.
The dream is to end the game fast. Snowball everyone and force a surrender.However, they usually wont, especially come ranked. To succeed late game, you must have reflexes. Using ult distortion is the best couse of action. W R Q E thrn slip the tbe first pad before you tske retaliation. Enemies may even burn summoner spells. In team fights, ult distortion for raw aoe damage.
Hope these tips help, and gl out there with LB.