I don't have a lot of experience with pu/pd
I really only used it when DS#1 started standing as he would repeatedly pull up and I would have to lay him back down. At first he would get more worked up because he thought it was a game but eventually he got the point and it worked well.
Are you talking about naps or night time sleep? At night you have more time to work with to get the message across. Naps are a bit more of a dilemma ... She is definitely too young to be giving up naps completely so maybe she has just reached a developmental milestone that she is dealing with ... I would try and be persistent/consistent with pu/pd and see what happens.
Sorry I don't have any thing else to offer
Let me know how it goes though!