Lu Wei,

If not, there is the possibility that somehow Thunderbird got
configured to just grab a single message per check ...

Does Thunderbird has that setting?

I don't know for certain, as I do not use Thunderbird myself. But it stands
to reason that it does, as that way you can tell it to grab more on
good-and-fast connections, and less on slower and/or bad connections.

Regards,
Rudy Wieser