PSA: Assassins aren't supposed to be able to kill an adc next to their support if they aren't fed
I feel like people in modern seasons have forgotten what an assassin is.
Hint: they aren't champions that jump into a teamfight and try to oneshot a carry, that's what a diver is - which is a class if fighter.
An assassin's job is to clear wards and lie in wait to catch a carry OUT OF POSITION so that they can punish them for it. Sometime's by splitpushing to force a carry to come clear the wave, sometimes by camping by a blue or red buff or some other sneaky strategy.
If the enemy support is there to protect the adc, then guess what they ARE NOT OUT OF POSITION and you would be foolish to try and jump on them and FORCE the issue. Unless you are mega fed there is just no way it would be reasonable for you to expect to be able to 1v2 reliably unless your plan is to instakill the support first and then turn on the adc if you have enough cooldowns. It seems that a lot of assassin players here though don't even want to take that concession of getting the support for free and backing off so that their team can push with a numbers advantage, they'd rather go for the LCS BIG HERO PLAY of taking out the carry directly while ignoring that their champion is simply not designed to be able to brute force that kind of play.
If the adc or mid is alone pushing a side line or they linger out of position during a rotation then THAT is when you are supposed to be able to PUNISH them as an assassin. Note that the phrase is PUNISH because you are NOT ENTITLED to instantly kill them any other time then when they are making a mistake.
If you want more reliable ways to force an engagement on an enemy that don't rely so heavily on them being out of position, thats what mages are for - they can fish for kills with long range cc and if they can tag a squishy target your team is able to follow up and finish them off. The more committed version would be an initiation tank like malphite who can see a couple enemies grouped up and ult them to start a favorable fight.
That's not your job as an assassin. Your job as an assassin is to play mind games with the enemy, and catch someone out using superior vision control and positioning. If you cannot find a way to make a pick like this then you should wait until halfway through teamfights to go in on a low enemy and finish them before they are able to be protected by their teammates, but this is NOT the MAIN PLAN OF AN ASSASSIN, and if you are falling back to this then you have already failed in your main job of your class and should consider playing a different type of champion if this is how you want to play.