I am certainly no fitness expert but I know when I wasn't following a healthy eating plan and just working out with my Firm tapes I didn't lose a pound! Nothing in a 4-6 month period (I started in February and stopped in August of 2003). I was so disappointed. Are you following a healthy eating plan or just working out?
I also know for me a treddie, exercise bike or just walking the neighborhood just didn't do the trick. I needed intense cardio and weights to rev my metabolism. Combining a challenging workout routine with a low carb diet has helped me tremendously.
Also, maybe you are losing inches instead of weight. Have you taken your measurements? I hope something I have said helps you. It's so frustrating to work hard and not see results!