NavMesh obstacle avoidance faulty?

I am making a tower defence of sorts.

In the picture below, agents come down from the top and are heading to a location south-east of this maze.

They manage to work their way through most of it, but then begin pooling at random areas of the maze, as can be seen near the bottom right. After this, they cease movement full-stop.

A path should be able to be found:

The black objects are "Nav Mesh Obstacle" objects placed at runtime.

Am I missing an obvious setting, or is this pathfinding inadequate for this type of task?