Problem when sieging castles: Troops get stuck halfway up ladder

Sometimes when I siege a castle a couple of my troops will get stuck halfway up the ladder, creating a bottleneck that other troops cannot get past. The troops that are stuck 'run' on the spot but don't move. Anywhere from 1-5 get stuck, median of 2.

I'm not sure how to fix this. I will call all troops to follow me, which brings all but those that are stuck away from the ladder. Sometimes this will work and the ones that are stuck start moving again, sometimes it wont. Sometimes it creates an easier path and the rest of the troops can charge up the ladder past the stuck ones. Sometimes the only thing that works is to shoot the stuck guys in the head, but when there are 4 or 5 this isn't the best option.

Any ideas?

I play native warband, tweaked to increase the number of arrows/bolts per bag.