Indexing on Outlook

Windows 10

Ms Outlook 365

I have recently upgraded my Outlook from 2010 to 365, and migrated all my old pst files to 365. My indexing says "index complete", but all the emails in my previous pst files have not been indexed. I've tried to rebuild indexing, removed/added Outlook in the advanced indexing option, but no of them worked. It would be appreciated if anyone could help with this. Thanks.