Alerts via headphones

Having now set up and used my Fenix 6X I have one question. I listen to music from my iPhone connected via Air Pods and have my Fenix connected to the phon via bluetooth also. Last week I was able to listen to Fenix heart rate zones and hourly pace through the air pods, now nothing. I have the alerts set on the watch so not sure why this is. Any advice would be most welcome. Thanks