I think you need to up the intensity of your exercise.
Becuase you reached this state by doing exercise at a certain intensity level. Now to go to next level you need to up the intensity so that the body is forced to shed weight.
Even I use Tabata for my workouts. The way I have uped my intensity is by increasing the load. In my case, I use a weight vest. I increase the weight loaded in the vest as I reduce weight. So every time the body reducesd weight, it will never be working at the same load level.
I am not sure what exercise you use. Assuming you are using body weight exercises, you can up the intensity in many ways.
One important point to be noted about the Tabata Protocol is that, its is not carved on stone. We need to experiment and tweek around it to make it work. When a group of subjects are exposed to a certain exercise, with a certain intesity level, the results achieved in terms of fat reduction and strength increase will vary from subject to subject. Its never ever the same simply because there are a lot of factors which going into these results.
So with these in mind here are my recomendations for increasing the intensity of your tabata workouts.
1. By increasing the reps during the 20 sec work interval.
2. If you cannot increase it any further, then incerase the rep interval to say 5 seconds more. Meaning instead of a 20 sec interval do a 25 second interval
3. Another way to up the intensity is is by reducing the rest periodic. Tabata recomends 10 seconds, you can reduce the rest to say 7 seconds...and later to 5 seconds may be
4. You can even combin (2) & (3) to make a real horror workout
5. Another way would be to do it is by multiple tabata rounds in 1 session. Like you mentioned, you do 1 Tabata in the Am and another in the PM. May be you can start with 1tabata..take a 5 mins rest and do another or may be 1/2. The same you can do in the evening as well.
6. If you are using some kind of weighted exercise, then increase the load a little bit (not too much). When you see progress increase the weight a little bit again...and so on.
I hope this helps. Please do post your findings if you are planning to test this out.