It's been happening to me this week too! I had a mini-break last week as I was on hols, then since Saturday I've stuck with the plan and lost some weight - about the same as usual - but the ketostix test still says no... Can't complain if the weight does come off anyway, can I...?
There are two types of ketosis, and the sticks only show up one of them - providing you have followed the plan you are definately in ketosis, only the one that doesn't show up on the ketostix.
Also you could be in ketosis but burning more energy than usual. I read that yesterday, so even if the stix say no it is a yes!!!! Especially if you are drinking loads!