Doorbell ring sound not working on phone at times

Hi all, My ring 2 doorbell sometimes fails to notify me on my phone when someone presses the door bell. This doesnt happen all the time but latley it has, the only thing i have done different is i have added a chime and it works perfectly when the doorbell is pressed. When it works properly when someone presses the doorbell the chime sounds and my phone also notifys me with a doorbell sound, but when it fails i can here the door chime working and i get nothing but a motion detection sound on my phone. As i said it works at times and other times it fails, any ideas what might be going on?