You know that exact thing happened to me. I tried Atkins in 2004 and my weight dropped like a rock. Then I gained it back unfortunately and tried it again. The 2nd , 3rd 4th time etc. it did not work I just stayed the same weight.
Now I am doing Paleo/Primal (which is similar in some ways). But the thing is now I am tracking my weight , food and calories, activity here on Fitday. I think I was eating too many calories on those other failed attempts
we will see but so far so good