There may be some London Drugs stores that limit the amount of stacking you can do at one time.
Despite the current policy from London Drugs' head office, you will get an infinite number of variations of it depending on the store and the staff person you're dealing with. Sometimes you will even get different results from the same person at different times. The possibilities range from no stacking at all to almost anything goes if you have a coupon friendly cashier. It's like trying to nail JELL-O to the wall! lol
I did a Dove shop a while ago to specifically test these $2 WUB2 Dove coupons out at a London Drugs store in Vancouver:
http://img217.imageshack.us/img217/9020/89372642.jpg
I bought a Dove Men antiperspirant and a Dove Men body wash. They were from two different product lines. I had five differently coded $2 off WUB2 coupons and one $1 off Dove Men body wash coupon.
What's funny with this purchase is that the cashier didn't bother reducing the amount of a couple of the coupons to bring it in line with the product price. So I actually got two cents overage. I think there's only one thing that is truly consistent about stacking, your results may vary. Keep trying, and if and when you find a staff person that is coupon friendly, stick with them. :)