Definitely wait until at least an hour into the smoke before spritzing to let the rub (bark) set a bit. Any earlier and you'll risk rinsing off the good stuff.
OK. so you're removing the membrane...that's good. My next advice is to let the racks come to room temp before placing in the smoker. Placing cold meat straight from the fridge onto smoker just "shocks" the meat and the results are often tough. This applies to direct grilling, as well.
Next... make sure your smoker is maintaining consistent temps of no more than 225* to 230*. Ribs cooked at higher temps cook to quickly and also become tough. These guys really benefit from the low & slow.
Lastly, if all else fails, try buying your ribs from a different source. Some retailers have been known to sell meats from older/larger animals for increased volume but often substandard quality.
Good luck on your next attempt, don't give up, your results will get better, guaranteed!
Cheers,
Brian