The key is to ask them to do it ahead of time rather than after the fact.
If they fail to do something they should do and then you "?" ping them and ask them why they didn't do it, they aren't going to realize you are giving them feedback and that you are asking them to help next time. They are just going to think you are flaming them for something that went wrong.
It is much easier and more effective to say from the get-go at the start of the game, "Hey can you guys peel for me in teamfights and brawls? I'm not going to be able to help you fight them if their divers or assassins get to me."
Most people will do the right thing after they realize it is the right thing to do. But they aren't going to realize that if they think you're flaming them instead of giving advice.