There are many potential causative agents for why a snake might exhibit the symptoms you describe. Parasitic loads of various types can be one possibility.
Simple factors like too large a food item, too many food items, too hot, too cold, too much stress, or a myriad of other "toos". The problem can start
from any seemingly innocuous reason and become habitual based on repetitive feeding attempts. The keeper feeds once, the animal regurgitates and the keeper tries feeding again too soon after. Stomach irritation, beneficial bacterial loss, enzyme shortage, may all be factors at this point and can only be rectified
slowly. The animal needs to be built back up. Pro biotics, enzyme supplements, minimum of 10 days before next feeding attempt at which point one single food item no bigger than the girth of the snake should be fed ( a pink will work). One very simple aid to provide live acidophilus is to dip the food item in plain yogurt leaving the head clean.
If your problem isn't parasitic in nature you have to slowly rebuild your snake. One single small supplemented food item spaced a week or more apart until it regularly holds those down.
I once received a young adult snake from somebody in a group of four that the seller told me could only keep down pinks.
The fact that the other three had no such problems led me to believe that the single snake could be mended and that parasites were probably not a factor. The snake in question seemed to regurgitate every other meal in my first few weeks of isolation and I couldn't figure it out. One day I realized that it was only spitting up when I fed it frozen and keeping down every live meal. Conventional wisdom would suggest that a frozen thawed item would be more easily digested but in her case, it appeared she lacked a digestive enzyme that she was borrowing from live food. She now eats as many live food items as I will give her with zero problems but it was a long way back.
Walt Deptula