i don't know, it's like they have a sensor on there backs that let them know when they are layed in their own bed.
both of my boys did this. which is why i ended up co-sleeping. which is much easier if you breastfeed. which in your other post i read that you do, so that may be an option, but if that's not for you, i understand. i have a friend that breastfeed and she didn't want to co-sleep and her baby wouldn't sleep in his bed, so her baby slept in his carseat by their bed for the first 2 months. she kept saying, i feel like a bad mom. i kept telling her don't, he sleeping fine, just not in his bed, he will eventually. it just took time to get him used to the bed, and now at 6 months he sleeps all night in his bed. so don't worry, she will eventually get there, just keep trying and trying. maybe you could try to put a recieving blanket in the dryer to warm it up and put it in her bed before you lay her down. this used to work for us during nap time. they like the warmth of our bodies, so it's less of a change of being put down in a cold bed.
good luck and take care