seller metrics - what can i do about it?

Question

so i have received an email stating my items not received rate is now high. the thing is i send everything immediately with royal mail mostly second class large letter untracked if under £20 and royal mail have been awful recently for both losing my items and late deliveries although my late delivery rate is ok according to ebay. what can i do to improve this as im now getting next to no sales through absolutely no fault of mine?

tech-tools-gifts · Accepted Answer

Thanks to all that took the time to respond. I’m truly grateful and my faith in this page has been restored and a big thank you to the eBay mods that are taking the time to reply to people since I posted. I’m humbled!