Second, every team member needs to provide input on what is reasonable to get done and to be clear on what the expectations are. If the team isn't bought in leadership failure goes up for individuals and the team. So let's say the team has bought in and you have managed expectations externally and not let the team overcommit. Now you have someone that isn't meeting the expectations on your daily/weekly validation steps. You now need to have a conversation about why and what is happening. Is it maintenance on another project or component that is causing it, are there other outside factors, or is the person just behind because of a lack of proper skillset etc.
So essentially, if you are doing this right, you get team by in, manage expectations internal as well as external and than you follow up regularly so you don't have a team failure or even an individual fail without plenty of attempts to fix it or prevent it along the way. And when you are doing this, you aren't approaching team members confrontationally, you are getting feedback, understanding by listening and than communicating. Sometimes people do need a little kick in the butt (so far I've never met anyone immune to that need at some point), but most other times people need to be heard, and have clear expectations.
If you follow this, the teams' confidence and velocity almost automatically go up drastically. Holding people responsible is absolutely required, but you need to first have clear and managed expectations as well as communication.