Make sure Bluetooth is activated on your smartphone and that you have followed the installation instructions for the iHealth Air (PO3M).
If you let the oximeter battery discharge completely, you must reconnect it to your smartphone before taking new measurements in order to update the date and time. If you do not do this, the offline measurements that you have taken will be synchronised at an earlier date (e.g., 01/01/2000). This may explain why you do not see your latest measurements synchronised at the expected date.
*credit to Sylvain of iHealth Labs EU