Open the concerned activity in Garmin Connect Web, verify if the power value is there all right. If so, then in the cogwheel menu, select the option Set as PR and set the Power record.
I have the same problem with my Garmin 530. I've got a new "Max Ave Power (20 min)". I've got the power meters on the bike pedal cranks linked properly so that data is fortunately displaying in my Garmin 530 unit but it is not transferring to my smartphone app or my online computer record. This is the second time I've increased this personal record and it has not updated either time. I find this very frustrating as it's not easy for me (a 62 year old man) to go 100% intensity on a category 3 climb for 20 minutes. And to do this repeatedly and not get it updated on my online records is very frustrating. I've no idea how to add them manually or if this is what I really should be doing. This should be done automatically.
Open the concerned activity in Garmin Connect Web, verify if the power value is there all right. If so, then in the cogwheel menu, select the option Set as PR and set the Power record.
I also have the same problem with my 735xt