The following image shows how I had my gate's event area defined previously. Not sure why, as I had motion detection properties all set on highest sensitivity, but it failed to capture every other event at my mailbox while adding way too many false positives from highway traffic.
I was finally able to resolve this after about a year of trying. My conclusion is that some events failed to trigger before because of lack of contrasts at mailbox, especially when sun wasn't strong. I resolved this problem by adding a 2nd event zone AND increasing the time duration of events to 2 seconds, more than the amount of time it takes a vehicle traveling 80MPH to pass through zone. I am so happy this worked and hope it helps others.