I'm one of those people who likes to wait until all my items are rung in before I scan my store loyalty card because I like to watch the $$$ fall after I swipe. If you are like me, just make sure your store card goes through before the cashier starts scanning your coupons!
I don't know if all stores are like this, but at King Soopers your coupons will not double unless you have scanned your SooperCard. Even my cashier today didn't know this! She scanned my order and I swiped my card but it didn't register it. She started scanning my coupons before I noticed what had happened. After a little back and forth she realized (and even said), "Oh you're right....I don't know why it does that."
I felt a little silly because the coupons that didn't double were my .15 off any Odwalla bar coupon but HEY it's still .15 and I had 4 of them. I'd like my .60 please! She was great and rescanned 2 of them to make up the difference and I was still able to get my bars for only .58 each. Not the most amazing deal on the planet but they are a good snack, especially for this pregnant lady. :)
So just note, make sure that store card goes through so your coupons double. Every little bit counts. :)