Ok, it's not my server, but it's something strange with FlashFXP.
On one client I had the server in my Site Manager and it works just fine. The other client I'm having problems with I was just using the Quick Connect and the URL history - this one for some reason does not send the RETR 0 and RETR 100 commands at the start of the session.
There's no option that I can see for Quick Connect to specify use resume, so perhaps it's a bug with SSL/TLS when using Quick Connect.
At least I can get around it now
