I had the same issue with Brendan. When he was little, he would start to fall asleep on the soother.. and would be lying with his eyes closed for well over 20 mins. but as soon as the soother fell out, his eyes would pop open. Or, the soother would fall out and he would get upset, and it would interrupt the flow of falling asleep.
To be honest with you.. and my apologies if this isn't helpful.. I ended up having to ditch the soother, and helped him to fall asleep with out it. It wasn't easy, I did a lot of pat-shush, but he would fall asleep for a longer period. There was a lot of tears.
If this isn't the way you want to go.. you might want to look at getting a different type of soother. Some nipples fit the mouth better.
Sorry if this isn't helpful. :oops: