Sometimes with USB to Serial converters they set the COM port to like COM16 and other stupid numbers, not sure if your software supports COM numbers this high so Id check that out - some apps only read COM port numbers to COM 4.

Every seperate USB port you plug the converter into will actually come up as a seperate virtual COM port - so try to always use the same USB port each time. You can force a COM port number re-assign via the driver properties in Device Manager (but be careful not to overlap, if you do a reboot should fix it), mines set to COM 4 and I always use the same USB, so its always COM 4.