There are a few retailers that mess around with the purchase date when they submit info to Rakuten so if it doesn't match up with the date you clicked Rakuten, the automatic addition of the cash back doesn't happen. For example, I buy today, but the retailer marks the purchase date as the date the item ships out instead (or marks it the day after the purchase, etc).
Coach and Coach Outlet are some of these. Now, the minute I get my purchase confirmation, I copy and paste the info from the email into the form on Rakuten (click help, find my cash back, enter correct purchase date) so that they research it and make sure it adds up. Ever since I started doing this, I never have problems anymore.