I have the same problem. If you take the cover off I can see a motor and when I change the resistance it springs into life and tightens what looks like a break cable going towards the fly wheel of the bike. When I pull this cable manually the resitance doesn't change. I expect this cable somehow manages the resistance and over time it has either stretched (probably not as max resistance level that could be reched would decrease gradually) or has come loose somehow (this would explain complete lack of resistance increase). I can't figure out how to get into the fly wheel to look at the cable though...
Andrew
January 2009