Is the old computer still available? I hope so. The built-in export tool causes numerous problems. The following steps WORK.

Here is the link for moving a profile in Windows when installed in default location:

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
8. start thunderbird and all should be there

Question: were the lost messages in an IMAP or POP account?

Thanks David. I eventually found the instructions you gave but I had to learn the hard way. You should save others a lot of work.

Thanks again, Roger Garman

You are very welcome. What you experienced is not uncommon.