I guess I missed this when you first wrote it - but did you get it sorted out?
I don't really have too much helpful info to add, but we're having something similar happening at my house. My 19 months old sometimes won't nap or will just nap very briefly - she's tired but she'll fight tooth and nail before giving in to it, like she might miss something (and we don't make things exciting when we see she's tired and really should be napping). What happens is that she can't last through the whole day yet (well, sometimes she does it without a problem, but normally she needs to sleep), so she'll conk out eventually. Usually too close to her bedtime - if we let her sleep, even if only for 20 minutes or so,she won't go to bed at her bedtime, and if we don't, she'll be grumpy and miserable.
I'm assuming it's because she's borderline ready to deal with a shorter nap or even no nap (she's never been much of a napper in the firstt place), but if you've found anything that works, I'd be interested to know.