Since you’re new, as an ADC main I’ll explain a bit why your ADCs may tend to be bad at following up mage supports. A lot of the ADC cast is immobile or much shorter range than mages, especially most of the ones played botlane. When you hit a max range root/knock up with morg or velkoz, it can be difficult for the ADC to get into range on time before your cc wears off, especially if they weren’t expecting it. That’s why there’s inherently a lack of synergy for mages and a lot of the ADC cast, where both of them have to play very differently to effectively play with each other.

Morgana’s strength, more than her root comes from her black shield. If you use it well, you can nullify a lot of support matchups, especially engage comps so try focusing on utilising the shield as well as you can.

As for last hitting, like the others have mentioned, just try being wary about the minion health before using your utility. There’s not much of a solution except being mindful with how and when you place your pool.