My 15-month-old is not a good nap taker. Any suggestions?

My oldest two children (5 and 3) took naps like clockwork, each took a nap at 10 am and then again at 2 ... my youngest will play in her crib for a little while making happy noises but she gets really tired around 5:30 or 6 because she refuses to go to sleep. If we put her to bed around 6 she wakes up again six, any suggestions? And please don't be negative. As I said, I have two older children who were regular nappers. I'm frazzled, please be constructive.