I don't know if this has been said before but it just occurred to me that small babies rely on their sense of smell to know their mother and of course their hearing.
So I was thinking that it might help if their cot smelled of their mother instead of smelling like new cot, new sheets etc. So I thought that it might be a good idea for people with babies who have trouble sleeping in their cots if the mum wraps the sheet around her body for a few hours if possible - I know that sounds a bit mental- so that it smells of her and anything else near the cot if possible.
Then I went on to think it might help if the mum records herself talking softly and soothingly or even singing and play that in the baby's room when it goes to bed.
I think thats really worth trying!