Can't download email

I popped a bunch of email headers-only (to Thunderbird) and now when I try to download the content, the email vaporises. I didn’t delete them on the server, but they may have been spawned during all the email mess a couple of weeks ago and gotten chewed up?

Anybody else with this problem? If so, does anybody know start and end dates for these vaporising emails? I need to contact a client to have them re-send (not looking forward to that…).

Any other ideas welcome. Thanks.