Ring Doorbell no Working when Button Pressed

We have 4 devices - 3 cameras & 1 mains powered doorbell.More often than not the doorbell works but quite often, it doesn't! Today, a delivery from Amazon, pressed the button but we didn't know! Fortunately he rang me and I was able to collect the delivery from him.While I was accepting the parcel, I pressed the doorbell and it didn't work! I checked the history and it stated we'd missed a call and the video of the Amazon delivery was there.The doorbell should have been heard on two mobile phones - both phones are connected to our local WiFi. Anyone know why?Thanks