I've read from many sources both online and in books that babies this age and older should get 2 naps a day, each 1-2 hours. However, I've also learned that at around 4 months and then again at around 9 months some babies go through a sleep regression where they don't sleep as well as they used to. It has a lot to do with brain development; their brains are so active that it's hard for them to calm down and sleep. They are learning so much! Other factors include growth spurts (maybe she is more hungry more often than usual) or teething (maybe she is experiencing a little pain).
All you can do is keep trying to get her to nap...more than likely it is just a phase that may last from 2-6 weeks. It's always possible there is something more wrong, such as an ear infection, that is causing her pain and preventing her from sleeping for long. My guess is, it's just a phase and will pass :)
at 2:07 PM on May. 16, 2011