Sam,
What flow behavior is the model exhibiting? How does this differ from what you expected?
I would infer that you are getting pooling instead of flow leaving the domain. I neglect to use a 'Boundary' at all for my M21 overland flow simulations. Instead, I create an artificial boundary by manually raise the elevation at all of the boundary cells to something arbitrarily large to contain any flow (or else you will get an error) and place 'Sinks' on the cells where you want an outflow boundary, setting the 'Magnitude' to something arbitrarily large to allow a free outfall.
I'm sure there is a way to do this with the boundary approach, but like many other aspects of this software, I'm not sure how. Maybe you will find this workaround useful.
ab