My midwife at my antenatal appointment was talking about this the other day - she told me that often with second and subsequent babies the head doesn't tend to go right down until the very last minute - for many women, it won't happen until they are actually in labour.
I meant to add before - my daughter did not engage until she was ready to be born. I was made to sit on the bed & not move for fear of a cord prolapse - when she did move down it was right before she crowned.