I've got no expereince of digital stepper drivers, but I've never had any issues with USB adapters setting up other drives. Could it be something like the USB/RS232 interface you're using not implementing the full RS232 standard?

Not all USB adapters connect or implement things like the DTR/DSR/RTS/CTS lines, which could be the problem.
I normally use FTDI based adapters, and any I've had implement the full pin set, however some do exist that don't.