Mr. Bubbles drowned his rat once, though I suspect it was unintentional. He snatched it up so hard, they both fell into the bowl. Unlike your snake, he kept it wrapped in there until it was gone instead of the repeat dunking. But he did need assistance pulling it out so he could eat it. Every time he tried to grab it to eat, it would just bob away from him. Poor guy looked so upset, it was to funny. But he made room so I could lift it out for him and started searching for its head as soon as I was clear.
I know that doesn't help you out at all, but I thought I would share.
To get back on topic, though, has your snake shown any signs of side tenderness or injury? I'm asking because maybe he's trying to avoid constricting for some reason.