23 mo old not napping

DD2, almost 2, is a great napper a lot of the time but the rest of the time, no nap at all. I'd say in a typical week she naps 4 out of 7 days. On the days she naps she sleeps between 1.5 to 3 hours. Today she didn't nap but it was weird because at 12:00 she was crying and begging me to put her down for her nap. This is about an hour and a half earlier than I would normally put her down but she seemed so tired that I did anyway. We'll a few minutes later I see her pop up in her crib on the monitor and hear her singing. I kept trying, even though it felt like a lost cause, for over another hour because she seemed exhausted. The other part of this is she's starting stripping naked in her crib so I can't just leave her in there to hang out, which is what I used to do. During this time I'm constantly going up there to put her pjs back on and to get her to lie down. Finally, I gave up and brought her downstairs. She doesn't seem tired at all now. She's playing happily while I type. She's got such a mellow temperament and she seems to do ok without a nap but I feel like she really needs one. Her older sister dropped her nap around 2 but she was never a good napper to begin with. It was such a battle every day that when she dropped her nap it was a relief for me. With DD2, I am really not ready for her to drop her nap and I feel like these kind of days could be avoided. Any ideas?