Hi I'll assume you've verified all the COM ports to use and have properly initiated a Console session and hit the enter key a few times. That said, what are the specs of the serial cable and converter you're using. I have just tested the standard Cisco RJ-45 to DB9 Console cable, and connected it to my Laptop using a Prolific Serial to USB cable and it works just fine. Extreme switches should easily connect with any standard console cable that has the following pinouts:
DB9 F: Pin 1, 3, 5 (10, 30, 50 (Black, Yellow, Green)) and on the RG45 side: Pin 3, 5, 5 (Blalck, Green, Yello) or . Obviously you won't be able to see the pinouts on a the DB9 side, but you should be able to confirm that they are as above on the RG-45 Side.