Drink coupons (4) are normally sent after each set of 10 paid (not points) one-way flights.
Assuming you're opted-in in your Rapid Rewards account (see below for details), you should contact Customer Relations and have them review. Perhaps your mailing address is incorrect, or there is some other issue. Are you receiving other mailings from Southwest? Like the Kick Tail slips?
How to check if you are Opted-in:
Log In. Click on My Account. Under My Preferences, click on Travel. In the upper right corner of that box, click on Extras. If you're "opted-in" it will look like the pic below. Otherwise, edit to change.
Make sure you have the drink coupons option on in your account if you do I would message southwest on twitter or Facebook if you have those and ask them for help as it will be much faster than calling customer relations if you don't have Twitter or Facebook you can email or call customer relations and they would be able to look in to this for you if the option is turned off be sure to turn it on and maybe try to reach out anyways and they might be able to send you some anyways for being a great customer.
hope you get those coupons as the price of drinks just went up recently