WritePort of the USB print monitor

Hi,
I am writing a custom language monitor for the printer driver.
Now I am experiencing problems with USB printers connected via standard USB
print monitor.
When I call from my language monitor a WritePort function of the underlying
USB print monitor it returns sometimes FALSE with 1223 error (The operation
was canceled by the user).
This problem does not occur with LPT port monitor.

The question is:
Must WritePort be called only within StartDocPort/EndDocPort pair for the
standard USB port monitor?
What to do if I want to issue WritePort call outside this pair?

Thank you.