DD was a shocking sleeper in her cot - I think she is like me and doesn't like small spaces...
At 15mths we were given a toddler bed so we took her cot out of the room and put her bed in. All day she played on it and that night she went down like a dream and has never slept better!!!
She does get up occasionally in the early hours of the morning but she just comes into bed with us - this happened maybe 1 out of 30 nights and is usually when she is sick...