How do I sync my MPA with my Garmin 735XT?

I’ve got the MPA and Power data field installed on my Garmin 735XT and sync the watch with Garmin Connect on my phone most days, yet the MPA value doesn’t reflect what my online Xert Profile is saying it is.
I have noticed that my MPA during exercise doesn’t match the chart once I’ve uploaded my activity to Xert e.g. my 735XT shows a breakthrough, but my online chart in Xert shows a gap between MPA and TP - is this linked to the lack of sync’ing?

I’ve just checked the App Management > Data Field settings and noticed the Username/Password creds were empty, so have re-entered and confirm they are retained.
Sync’d the watch whilst on the data screen using the MPA & Power data fields, still haven’t updated though.

The watch needs to be connected with your phone and you need to start a recording. After that, the process can take up to 5 minutes. If it is successful or there is a problem, you’ll see a pop up message.