I submitted a promo code, but I am told it's invalid. Why?

There are a few reasons why when you submit a promo code, it won't work.

The promo code has expired. Promo codes are only valid for a certain amount of time, which could be hours, days, or weeks.
The promo code was already entered. If you previously submitted the code, you won't be able to earn points for submitting it again.
The promo code reached its claim limit. Sometimes a code will only be valid for a certain number of submissions, and when that limit is reached, the code will no longer be available.
You misspelled the code. Check to make sure you are inputting it correctly. Codes are not case-sensitive.