With the Spotted Python, it sounds as though the reason it is not eating is because it is stressed. The fact it strikes but does not hold onto the prey would suggest it is striking in defence rather then a feeding response. Try and disturb the snake as little as possible when attempting to feed. Also, offer the pinky on the end of long forceps so the snake doesn't know you are there. If possible, do not remove the animal from its hide when feeding, try and get the lid off the tub without disturbing the snake, then using forceps, place the brained pinky in front of the entrance of the hide and move it around to try and get the attention of the snake. Try to have the head closest to the snake so that it gets a good taste of the blood. Sometimes the snakes head will be at the back of the hide and it won't notice the pinky at all, so it may take several attempts before you have success. Also make sure the pinky is as warm as a live one would be.
For the Darwin, wait until you see him out and about during the night. Again, try and open the tub without the snake noticing, and using forceps present a brained fuzzy to the snake slowly. Once he starts tasting it you may need to move it around a bit to get him to strike.
The key for shy snakes it to be able to present the food to the snake without them knowing you're there. It takes a bit of practice but you'll quickly learn what to do, and what not to do, judging by the reactions and the stress levels of the snake.
