How can I make RVOController units route around each other?

I have a scene where two groups of units meet to do battle. Assuming both forces are marching single file towards each other, when the two front ones meet, they stop moving and start fighting. The two behind should flow around the ones in front, who are now stopped, in order to be able to move forward; but they don’t. I find that so long as all units are moving, they avoid each other just fine. But if one of them is still, the other will crash into it and not try to go around it.

Any ideas what could be the problem?