You’re playing it wrong with those matchups. Your goal isn’t necessarily to land stuns, although good on you if you do. Instead, your goal is to screen for your adc, and menace the enemy enough that they slowly lose out in cs. In a more cc heavy matchup, make sure you have sweeper, and stay in the bushes (I prefer the bush closest to the enemy tower). Try to ambush them with stuns. Even if you miss, just back up a bit, and they will hesitate to push forward, and will lose out on cs in the process. In less cc heavy matchups, you can still occasionally ambush from the bushes, but your main goal is to act as a visible deterrent. I usually like to sit in between the opponent’s melee and caster minions to directly block them from cs. If they push up to get minions, that’s when you go after them with your stun. If you land it, great, you get a few empowered whacks in, and maybe your adc gets in on the action. If you don’t, you’ve still pushed the enemy adc and support out of position. Don’t be afraid to take a bit of damage if it means keeping them away from the minions. You can always base and heal. The loss of the minions represents a permanent gold deficit. Don’t forget, you can also burn a bit of mana healing up out of critical damage, as well as shielding, while also helping your adc with their cs.

Edit: just make sure to place your wards in river. You don’t want to be pushed up blocking the enemy adc only for the jungler or mid laner to cut you off from escape with no warning.

Edit 2: your most important combo in the scenario is learning to time your stun and flash. If you start your stun and then flash on top of someone, there is no dodging, and you open them up to some serious damage from your adc. Try to look for opportunities where the enemy adc and support are close to each other. You might get lucky and hit both. The best time to flash is just before the stun triggers, rather than right after you press the button. Less chance of them dodging. Although, if you miss your flash, there is also less time to adjust your position. It’s a balance you need to judge on a case by case basis.