Sorry kamots, no magic answer. We can ask all the normal questions about smoker temp, grade of brisket, etc, but we can't always find an answer.
That's why (I'll be you know what I'm going to say...)
It's done when it's done
Just means that whatever we're cooking sometimes has a mind of it's own. Recently I've done a lot of brisket and ALL of them are getting done in less than an 1 per pound. But that's the new FEC cooking a little hotter.
If you know the temp is what you set it at, and it's a similar size to what you've cooked before, sometimes, it's just a mystery.
Keep good notes and see how the next one does.