What did I do wrong? And, more importantly, how do I avoid it again?

[pjx2-2]

In Jan, I made top seller.  In Feb I was made "above standard".  In Feb I needed to refund on a sale that USPS tracked and lost. In Mar I was still above standard. Right now if evaluated I would be top seller.  What did I do wrong in Feb or Mar   AND   HOW to avoid it in future??