There could be several reasons why the "FE Custom" promo code didn't work. It's possible that the code is product-specific or requires a certain cart value. Sometimes, codes are only valid for certain items or require a minimum purchase amount. Additionally, the code might have expired or been used too many times. It's always a good idea to check the terms and conditions of promo codes. If none of these reasons apply, it's possible that there was a technical issue with the website at the time of use. If all else fails, reaching out to FE Custom's customer service might provide more insight.