Is it because the circumstances in which he went to sleep have now changed?

I am trying really hard to put DD in the cot awake, so that she can resettle herself...it only works about 60% of the time though (through the night strangely and only once or twice in the day).
Hang in there hun. You're doing a great job. My midwife said some wise words to me..."don't chase sleep because you'll never win"...I found that really helpful on the days DD would not go to sleep. Try to be consistent and I'm sure you'll see some improvement.

HTH