Migrated some mail

Hi,
Sorry if this is covered in another thread. I did search.
I have just migrated a significant amount of emails. I can see them all in Thunderbird and in the old web based RB site. They are not all visible in RB7. Also what’s odd is they aren’t all visible on my iPhone using the Mail app or the RB7 app. Any thoughts? It’s really sporadic.
Thanks