HRM-Pro Plus bluetooth and Garmin Connect cannot sync

I have had my heart rate monitor strap for 1 month. I sometimes wear the HRM-Pro Plus (version 8.80) without the watch (e.g. when the watch is charging) and I want to synchronise the data via the Garmin Connect app. The phone recognises the strap via BT, Garmin Connect does too. The problem arises when I need to synchronise the data. Garmin Connect says connected and starts syncing data, then 5-10 seconds later it says Not Connected. Sometimes the sync doesn't even happen. It just says Connected, and then a few seconds later Not Connected. And so on and on, trying to synchronise for e.g. half an hour, but failing. The last time I managed to sync was 14 days ago. If I use the strap via ANT and the watch, then I have no problems. I have already removed the HR belt from Garmin Connect, reconnected the belt to the phone and nothing has helped. Does anyone have any idea what could be wrong?