If a sugarplumlane.com coupon code isn't working, several steps can be taken to troubleshoot the issue. First, ensure that the promotion code is being used correctly. This can be verified by reading the terms and conditions associated with the code. Not all items may be eligible for promo codes, so it's important to check if the code applies to the items in the bag. If the code is not recognized, check for any spacing, character errors, or spelling mistakes. The promotion code field is case-sensitive, so ensure that the 'Caps Lock' button is not on and use upper case where necessary. Also, avoid mixing up the numbers 0/1 and the letters O/I. If the code is still not working despite these checks, it may be necessary to contact customer service for further assistance. Remember, if the code is expired, it won't be possible to use it anymore.