Check the expiry date on the strips. I had a pack that were just one month short of it and they never seemed to work. The new pack, with about 2 years to go, are fine.
There's no chance they could have got a little damp in the pack? Stored at too high a temperature?
Lastly, it might be necessary to wait more than just a couple of days, if you've had a lot of carbs previously? (I admit this last is just guesswork.)
Also, check if you're losing weight or inches (or even millimetres) and if you are then don't worry about what the sticks do. I think even Atkins said some people never see the colour change but still eventually lose the weight.
Good luck

Zan