Well I guess whenever I have that problem, I just stop trying to get her to nap. I just go about my day and do what I want/need to do so she's kept busy and won't get too grumpy even if she's tired. Eventually in the car or nursing she'll just pass out without a fight. Then she seems to find her own 'schedule' again and will sleep. And I don't go crazy spending time obcessing over when she will nap.