Some emails didn't import from Thunderbird

I recently imported from Thunderbird on which I have a couple of email accounts dating back years. All seemed to go well except that for one of the accounts the Sent folder only imported the most recent month or so of emails (a tiny fraction of the total). This isn’t a local folder (so it’s maybe also odd that it hasn’t filled them in since by syncing with the email server, not that I know much about this).

What should I do?

Otherwise I’m happy with eM and will probably switch over entirely and get Pro if I can solve this!