Skipping naps

Lately napping for my daughter has been hit or miss. Some days she'll put herself down and some days she won't nap at all. Some days the only way to get her to nap is if we drive around and she falls asleep in her carseat. The past two days she hasn't napped at all. It doesn't really seem to affect her, she's maybe a little more whiney around the end of the day but not much and she goes down for bed within minutes and sleeps all night. I know sleeping is important still for them at this age but I don't really know how you can force a kid to take a nap? If it continues I plan on calling her pediatrician to see what they recommend but just wondering if anyone else is dealing with an on and off napper.