If Your Baby Doesn't Fall Asleep for Nap Time, but Isn't Crying Do You Go in and Get Him Anyway?

I'm just curious, if you put your baby down for a nap and he/she doesn't fall asleep but stays in the crib and is playing by himself quietly and happily, do you leave him in their to play or do you go get him and try for a nap at a later time?

Suggestion:

If she's still awake after half an hour, not crying, then I just get her up and try again later. I do the same with bedtime. I figure if she;s not tired, she's not tired!