Now, that is hard when you're trying to get the children out the door, you're in the kitchen, and they're upstairs. You're shouting for them to get ready.
That's not as effective. It's easy for them to tune out. Think about it; if they were shouting at you from across the house, it's easy to ignore it and think, "Well, they will come and tell me face to face if they really want me."
You need to get present. So go into the room, speak to them and turn off the TV if they're watching it, or ask them to pause their game or whatever they're doing to have your attention.
It's important that they look at you when you speak, ask for what you need, and then get confirmation. You will know they heard you if they say, "Okay, Mom," or "Yes, Mom." They might need coaching to do this; it's common to have to occasionally say to children, "Please say 'Yes, Mom,' so that I know that they've heard me, and then I won't have to keep repeating myself."