Ok, based on this, it looks like the leads are syncing but the campaign statuses just aren't getting updated.
If the SFDC campaign are connected to marketo programs--even if the new leads are not yet members of the Marketo program--you must change your flow step to "Change Program Status" and find the program that is linked to the SFDC campaign you want to add them to.
We ended up manually changed the SFDC campaign members' status in SFDC. It works and members showed up...
For this case, I think it maybe because how we imported those leads into Marketo, the columns and data values in the excelsheet. I have no problem to sync them into SFDC, when I imported more leads in a different SC.
Thank you for your help here:)