EDIT: This is no longer available.
Some people commented last week that they haven't been getting theircoupon despite signing up multiple times. The fine print says only four coupons can be sent to any household during the promotion, so maybe a family member or roommate also signed up and together you all reached the four coupon limit without realizing it.
I bet there are other possible reasons as well though. A sneaky mailman. Computer glitch. Worldwide conspiracy to take over the world one stolen coupon at a time. They all make good sense to me!