Heart rate data not syncing/cannot manually upload

Wearing 735XT to monitor my heartrate through the day and night, but data not syncing with Garmin Connect on iPhone (even though other activity data is).

Thought I'd try to manually upload to Garmin Connect online. Setup new dashboard for heartrate but can't find any files on my watch to manually upload, firstly, because it's not obvious from all the folders on my watch which folder I should get data from and, secondly, because none of the folders appear to have a recent 'date modified' date.

Can anyone suggest a solution?