[Wine] Real Serial port vs USB Serial adaptor difference?

[k4king]

When printing some labels with the Seiko Smart Label Printer software I've come across a difference when using real serial ports vs a usb to serial adaptor. 
Printing labels with not much content is fine. 
But when printing labels with more and longer lines of address via a real serial port only about half the label prints. 
There is no obvious errors encountered and from the Queue manager of the SLP software it appears it thinks the label has been printed.
If I print the same label via a usb to serial adaptor it does print fine.
I have tried adjusting the speed and the various bit settings and handshaking in both Linux and in the SLP software on the real serial port with no improvement.
The same real serial ports work using the Linux SLAP software (though it is painfully slow to print).
And the same real serial ports can also be made to work with Hylafax without issue.