You have to be very careful with your words when telling people what to do and telling them what they did wrong, it's easy to upset someone. If you say "why didn't you do xxx?" it can come across like you are telling them off and it might be the case that they were really trying hard to do as instructed but simply made a mistake. A good way to help would be to remind people of the plan without calling out anyone in particular: "ok good try, don't forget to xxx".