Your first attempt sounds a lot like my first attempt at babybacks on the WSM. Took them off after checking the temperature at around the 4 hour mark. The meat was tough as all get out. After a couple of ribs the whole batch went into the trash. Wish I'd read this post before making the attempt.
One other possible reason may have been that it was a pretty big slab of ribs. Long enough ago that I don't recall the weight. Maybe the hog had been too old and tough for babybacks.