That the the code being used between the validity dates set, i.e. after the start date and before the expiry date.

That the code is being used against a product or service for which it is set to be valid. For example, has the code been set to only be valid at specific classes or on certain packages instead of all of them?

If the code is limited to be used by specific customers, is the customer trying to use the code on the list?

Has the usage cap for the code been reached? If the code is capped at 1 per customer or has a total usage limit it will not work once the limit has been reached or a customer has already used it once.

Is the code restricted to a certain number of places being booked?

That on input there isn't an unwanted space at the start or end of the code as the system will check each character.