Try assist feeding a few more times. I had a female pastel this year that I had to try 5 or 6 different attempts before she took it. Now she is eating on her own. At least with the snakes I've had to assist it seems like it gets a little less stressful for the snake each time. I think the stress is the reason they are spitting it out. I also hold the snake for several seconds after I put the mouse in there mouth so they will calm down before I put them down. I have had a couple eat in my hands because they started swallowing right away and I was scared if I moved them to much they would spit it out. Keep at it. It's so worth it when you see a thriving little snake that you know you brought back from near death.