Not committing to the breakdown on defense
The first decision for defensive players at a tackle situation is whether to challenge the breakdown or to ignore it and establish a position on the defensive line. It's worth noting that this a decision that will be made by multiple players (as many as 4-5) on each team at every tackle situation during an entire game. Getting this decision right in a huge majority of cases is essential to achieve success on defense.
If the defending team gets this wrong even 10% of the time, that would mean that someone is out of position at every second tackle situation. Think about that for a second. For your team to be successful in this area, they need to be making the right decision collectively at least 90% of the time. That's a very challenging proposition unless you have clear guidelines for who is allowed to compete for the ball at the breakdown and how they are allowed to do this.
Luckily, the attacking team can make this decision much easier for you with their support play. For example, if the attack's first supporting player gets to the breakdown quickly and establishes a strong position over the ball, the best option may be to leave the breakdown alone and defend at the Post, Guard or Monster position depending on the team's defensive structure at the breakdown.
If the attacking team's first supporting player does not have a strong body position, over the ball, there is an opportunity for the defenders to counter-ruck by trying to get underneath the shoulders of that supporting player to drive them off the ball.