Assist feeding after only 1 month might have been premature - depending on the snake's condition. More information and pictures could probably help people give you sounder advice. Hold old is the snake, how large, what was it feeding on with the previous owner, what have you offered, how have you offered, how long are you waiting between offerings, how much are you messing with the snake in general? A ride in the car to the vet and examination is stressful enough that I'm not at all surprised your snake didn't eat 3 days later. That scenario alone is making me think you're probably just trying too much, too fast. That said, the longest I've had to assist feed a snake was probably in the 7-8 month range before it started feeding consistently on it's own.