Mailbox Sensor unreliable

I have a mailbox sensor mounted in the rear of a plastic mailbox. The app shows excellent signal. However, the thing either never goes off, or goes off for no reason. I am constantly messing with the sensitivity setting, and sometimes it will work reliably, but lately, like I said, it either never goes off, or constantly goes off for no reason. No, it isn't being triggered by cars going by. There is literally no reason for it to go off, but it does. Help?