My little 5 week old wont stay asleep once i put her in her bed?

i can only get my baby to sleep by rocking her but as soon as i put her in her bed she wakes up, and then i have to get her back to sleep and the same thing happens again, this usually goes on for a few hours and then its feed time!! so by late afternoon she is so overtired it can take me hours to get her to go to sleep. as i also have a very demmanding 20 mth old so it is very difficult to spend this much time getting her to sleep. she is fine during the night as she co sleeps with me but unfortunately due to my other child i cant have her in my arms all day!