B.O.
I have 3 toddlers that have all gone through phases of sleep resistance or anxiety of some sort. Every instance the child had a different reason for staying up. One had a bowel problem that requires medical treatment, another had started to get night terrors and was afraid to sleep and the other was just plain stubborn and wanted to test me as far as she could. You have to first figure out what is causing the behavior and then decide if the solution is medical, emotional or behavioral.
For the most part, so long as I keep my schedule up and give them a solid routine (and stick to it!), and everyone is healthy, everyone naps at the same time and goes to bed at the same time. (they are all asleep right now...and I can play on the computer!)