I smoked one rack each tonight of Spare and Baby Back. I smoked the spares for 6 hours and smoked the baby backs for 5 hours. 225 degrees the whole time. My spares has nice pull back and we’re perfect. However, the baby backs didn’t really have much pull back and seemed like they could have cooked longer. Should I have gone 6 hours on the baby backs too? I thought they cooked faster than spares. Smoked on my Pitboss and didn’t wrap at all on this smoke.
Try wrapping the BBB's. I smoke mine at 225 for 2 1/2 then wrap them for 2 hours with some apple juice. They are fall off the bone. I know a lot of people here don't like it that way, but that's how I like mine.
Typically BBR's take an hour less than Spare Ribs when both are Wrapped. Cooking naked is different. You don't have the moisture to speed the breakdown of connective tissue. I find at 225-250, naked ribs need an EXTRA hour to get tender. It is likely your Spares got done fast and the BBR's needed more time. Lots of stuff can change cooking times. Age of the pig, thickness of the meat, placement on the racks. No guarantees when smoking meat...JJ