They give good coupons to people who just started or havent spent a lot. If you already spent a lot they think you'll keep on spending so no use in giving you codes.

The way I get $20 of $50 is...

Logging into my account, and changing my address. (IE, 1234 baker street to 1235 baker street)
Create new email on yahoo.
Create new account on tower with your real address, new email, and the name of someone else in your family.
Sign up for the e-mail newsletters.
Then, they should send you a $20 off $50 order through email as an introductory offer for "new members".