Judges 21:8-9

‘And they said, “What one is there of the tribes of Israel who did not come up to Yahweh, to Mizpah?” And lo, there came none to the camp from Jabeshgilead to the assembly. For when the people were numbered, behold, there were none of the inhabitants of Jabesh-gilead there.'

A check was made of the tribes and sub-tribes and it was discovered that the people of Jabesh-gilead had failed to respond (Gilead had a reputation for failing to respond to the call to arms (Judges 5:17)). And it was not a failure involving only the confederacy. They were seen as having directly refused to obey Yahweh. Such a failure rendered them liable to The Ban in accordance with the oath taken at the assembly. It was always a risk to refuse to respond to the call to arms (compare Judges 5:23).