As we all know it's awesome to work with a buddy and we should never try to take on the jammer alone all the time. Break down the wall concept to making each blocker accountable for their self, for their own body.
Drill one - using names
Skaters in groups of 4 - three blockers and one jammer. The wall has to block the jammer. The key thing here is to focus on using each other's names, rather than just yelling IN IN IN or OUT OUT OUT. This is likely to get a faster response from the skater you are trying to speak to and encourages skaters to be accountable.
Drill two - strong hips
Skaters in groups of three. Two blockers facing derby direction without a brace. The purpose of this is to learn to pivot BELOW the hips. To be completely aware of where every part of your body is at all times. The jammer will hit each blocker's hips individually and the blocker must hold to the best of their ability WITHOUT turning hips, hips must be squared off pivoting only from the knees. Jammer hits each blocker on both sides and then switch roles. Rotate so each skater is the jammer.
Drill three - doing it wrong
Switch groups of three. The same drill as before but blockers to "do it WRONG" on purpose. The jammer comes through and hits each blocker on each hip and the blocker should roll and move forward.
Once the jammer has gone through the wall with the blockers failing, they will go back through again and the blockers will hold correctly.
If skaters know how it feels to do it wrong - it can help them to be aware of their body and do it right.