I too have had to deal with a child crying all day. It was the younger sister of a boy that I had for 3 years already. He was the easiest child, but his sister? She was a year old and cried bloody murder for at least 3 weeks. She was a serious momma's girl and I thought I was going to lose my mind! Well she is 5 now and you would never know that she was like that. She is like part of the family and is so comfortable here.
You have to give them time to adjust. Some kids are easier than others...that's just part of the job.