Now she is learning to fall asleep in the crib, but she hasn't learnt to "resettle herself" into a new sleep cycle, yet that can take longer to "perfect"... I would suggest posting on the "nap" forums as their are people there who are more "clued up" on nap problems than I am... but I think if you want longer naps you need to try & resettle her again for another 30+ minutes & evenutally they "work it out" & do longer naps.
HTH