Sort Direction Not Working

I'm using this to sort by direction:

obj.setSortProperty('direction', dir);
obj.setSortProperty('index', idx);
obj.refresh();

I also tried this:

obj.sort(idx, dir);

Either choice appears to sort because I can pull the direction and index values back out of obj just fine after the fact, but the rows don't acutally visibly sort.

Any idea why it's not working?