I wasn't trying to be hurtful, please don't misunderstand. I just assumed you posted for advise, I'm simply trying to help. Perhaps you should go back to your original recipe. I wouldn't change my recipes after one complaint, you can't please everyone-some people are used to box mix cakes and Walmart cakes. It took me a long time to refine my scratch recipes, they are moist and yummy, after LOTS of trial and error. I imagine as a scratch baker you probably went through the same thing.
You've been in business for 4years and recently are getting complaints so logically I'd say go back to your old recipes if they served you well.
Maybe they wanted a free cake, some people are like that. Maybe they don't like the new recipe. You would know better than me
I understand how a complaint can hurt, it can be utterly heartbreaking but as a professional you've got to move past it and try to learn from it or let it go. That being said, trashing you on any level is childish, especially if you've done everything possible to satisfy them.