Okay, so I had the same problem. I've been there

For the past 4 weeks, my LO was waking up about 1-2 hours after I put him down for bed. But I think I finally figured it out because the only 3 times in the last month that he didn't wake up were the times that I didn't nurse him to sleep. So, I tried nursing him a bit earlier or keeping him awake while I nursed, then I would put him to bed as usual with the full sleep ritual, and ta-da! It's worked for the past week!! Just be prepared becuase the first night he cried when he was in his crib, but I ddin't go in and he was out within 8 minutes. The next night was the same, only he cried for 5 min... the next night, no crying. My theory is that he is now much more aware of what's going on, and so if I put him in his crib sleeping, he would wake up screaming because the last thing he remembered was being with me... so, a little separation anxiety I guess. Hope it helps!