To me it's starting to sound like meat source issue. Where and what kind/type of ribs do you purchased. For me, I found that I couldn't purchase any ribs in a normal store. I have to find completely un-enhanced ribs, otherwise mine tended to come out tough.
I now never have tough ribs, never ever. You're welcome to try some of this:
1. Remove membrane the night before the cook.
2. Soak over-night in a mixture of water, apple juice, and some worchestershire, and of course on ice...
3. Morning; remove from soak, pat dry, and apply the rub about an hour before placing them on the smoker.
4. On smoker at 275+-. I use hickory smoke. At 1.5 hours I remove them and put them in foil, with a fairly heavy spritz of apple juice and worchestershire before sealing up the foil. I cook them in foil for another 2.5 to 2.75 hours, give or take. They're done at that time.
Oh, forgot, I cook baby backs. At 4.25 hours, they're right at falling off the bone, but with any luck not quite there...
Hance - Lake Dogs Cooking Team - MiM/MBN/GBA CBJ and comp cook
Lake Sinclair, GA (strategically about an hour from darn near anywhere)
Started competing in chili cookoffs back in the 1990's and have competed in more than I care to count. I became a CBJ in MiM in 2005, then MBN and in GBA in 2010. I've probably judged 130+- BBQ comps (sanctioned and unsanctioned) over this time. That said, I really enjoy competing more than I enjoy judging, and hope to get back to doing 4 or 5 a year in the near future.