cry it out method

I didn't want to do cry it out method but I ended up having to do it with my 16 month old when I had her baby sister...but my confusion with it is at nap time I put her in her crib awake and she screams and cries for like 10-15 mins and then she goes to sleep...but we have been doing this for almost 3 weeks and she STILL cries...I thought that by now she was supposed to be okay to fall asleep on her own without hysterically crying....