If a decision is not checking off, then there is probably something required that haven't been done for that decision.
Read the decision tasks and hints carefully and make sure you have all that is required. Also, take a look at your round tasks on the side of your screen when you are in the PPC dashboard, email campaigns page or the landing pages page. These round tasks will be things you must complete before being able to run the simulation.
If the decision has a requirement to review information, simply clicking into the decision and out will not check it off. You will need to sift through the data and review the content.
If you have tried all of aforementioned steps, including refreshing the page, and the decision is still not checking off, please reach out to firstname.lastname@example.org.