Hi neighbors! This should not be happening, as your motion detection is designed to catch things within your motion zones and range. Many neighbors have reported missing a motion and finding that a recent motion occurred just before the missed motion. If you have motions around the time of a postal delivery, but not during, please check your Motion Frequency option in the Ring app. Check out our help center article for more information on motion frequency.
Adjust motion settings for optimal detection is always the best first step. Another variable to consider is wifi connection efficient. If there is wifi interference delaying connection, then the reaction or communication of motion can too vary in results. Check out our Community post about RSSI for tips on optimizing this signal strength.
If you have another Ring device on your property, check to see if this detects motion during deliveries as intended, for comparison with your Doorbell. If you have any stairs leading up to your entry way, check out our Wedge Kits on the accessories page at Ring.com. I hope this helps!