Discounts

[uptime100]

Why are we offered discounts are never given.  This is my second discount failure.  I was suppose to enter a code to get a discount when I paid with PayPal.  Clicked on confirm and pay and never was allowed to enter the code an no discount.  No way to complain or enter after the fact.  Here is the discount

Code was CMOTORS115 for $20.00 off on a $116.00 purchase of tires before 11/13/2015.

Is there some way to complain or follow up on to collect the money.  Any ideas