Make sure Bluetooth on your phone is activated and that you have followed the instructions in the article How to synchronise measurements in memory.
If you let the blood pressure monitor 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 unconnected measurements that you have taken will be synchronised at an earlier date. This explains why you do not see your latest measurements synchronised at the expected date.
*credit to Sylvain of iHealth Labs EU