Has anyone had much experience with serial flow control in twisted?

I've been using a twisted serial port in my gtk+ software for quite a while now, and I can get xonxoff flow control to work, but for some reason, rtscts flow control isn't working (or I may be using it wrong).

I'm connecting the software, via a usb-serial cable, to a Datalogic barcode scanner cradle.
I've been able to replicate this problem with pyserial as well.

Without flow control on, I can for example, send a particular serial string to the cradle, and the cradle replies instantly with it's software version (as a string).
If I scan a barcode, this also instantly shows up in twisted/pyserial/etc (as a string).

With xonxoff flow control on in both the software and cradle, the serial connection works exactly the same as having no flow control on.

With rtscts flow control on in both the software and cradle however, the cradle doesn't reply instantly with it's software version when the correct string is sent.
However, a scanned barcode shows up instantly in the twisted/pyserial software, and if you've previously requested the cradle software version, the software version string only shows up directly after receiving a barcode string.

Am I doing something wrong, or does anyone know what might be going on here?