I would suggest re-examining your diet for hidden carbs. I've found that many new "low-carb" products flooding the market are often saboteurs. They often include hidden carbs that can blow ketosis during the initial phase. Low-carb tortillas are one such product (they often subtract fiber to get their advertised count). I'd avoid them as much as possible during the first week. Also, don't really trust the sticks. If you're dropping weight, it's working.
I've also found that my body gets wise to low-carbing, and is more resistent to ketosis than the first few times. Some people are also just more resistant. It sucks to be on a 15-20 carb/day diet and not have it do anything - I know.