Hi there,
When our twin boys were little (they are now 11 months) we would rock them to sleep. A hand on the back to stroke or gently rock and they would soon be asleep. As time has gone by, whilst one baby will now self sooth if left, the other still requires stroking to sleep.
He's gotten desensitised to it to such an extent that it can take between 30 or even 90 mins for him to go to sleep like that. If left alone in his cot he screams the place down swem