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 everywhere)