Justin, that's what makes this so challenging (even
with our "Lazy Q" advantage)! BBQ is, by nature of so many factors, unpredictable! We come up with guidelines,
but they are just that; guidelines! Every piece of meat has it's own agenda, and no matter how "exact" we want to be, it sometimes says otherwise!
Now, I agree with Kari, that 6 hours was too long, at 235, for baby backs. But, the fact they weren't done to pull easily from the bone, surprises me. Which, leads me to believe it may not have been your technique, but more
the ribs themselves. It's hard to gauge one smoke's results as "the way it is." Unfortunately, meat is made by animals, and sometimes their quality control sucks!
Check your source for ribs, and maybe try another time. Try 225, with a water pan on the floor next to the smoke box, and start checking sooner. Experimentation, my friend, will get you "dialed-in" in no time!