Tennis elbow takes a while to heal. It's hard to say what's happening without knowing more about the severity of it and other related things you are doing like resting, ice, anti-inflammatory creams, weight training, equipment changes, etc.
I use the green Flexbar also. I do the exercise where you twist your wrists and the strengthening exercises for your forearms. I think it's been very helpful, not on its own, but as part of an overall solution to addressing tennis elbow. In my case, equipment changes were necessary because I was using poly strings and a stiff racquet. Even now that my elbow is healed, I still can't go back to the old equipment. In addition to that, I used ibuprofen, anti-inflammatory creams, ice after tennis, and added new exercises in the gym. I also tried a couple of those bands you wear next to your elbow, but I didn't think they gave me much benefit. After about 2 weeks I had noticeable improvement, and my elbow continued to get better gradually over the next few months.