Here's what worked for me--I had a few slow starters this year.

I gave them about 6 weeks past their first shed to eat...after that, I started the assist-feeding. I didn't wait until they look thin or start to weaken. I used rat pinks--my theory is that I want these difficult boogers to be on rats from the start, that way they don't wind up being mousers.

I used pre-killed, but f/t would be fine too. Wet them down, gently insert them deep into the snake's throat--if you don't get them in there far enough--partway down the throat--the snake will just spit them out. Put the snake down in its bin, and stay very still and quiet. Some will spit the food out anyhow, and you'll have to try again. After 3 attempts fail at assist-feeding, I cut the head off the rat pinkie and force-feed the head. I only had to do that with one animal. (I had 3 slow starters out of 26).

I do the same thing again in 5 days. 5 days after the second assist-feed, I offer a live rat pink. If it's declined, they get another assist feed. With the exception of one stubborn hatchling, they all accepted the live rat pink. I think having food going through regularly stimulates their appetite. (That girl eventually took a rat fuzzy. She's now eaten twice voluntarily, but not twice in a row--we'll see how it goes next week, lol).

As long as you're gentle--remember to be careful with their jaws, and be sure their airway is clear--and you don't spend all day trying it, they will be fine. There is a lot of concern over how stressful assist-feeding is...and it IS stressful, and they can be exhausted afterward--if the hatchling is otherwise healthy, it won't really do them harm--they will start to put on weight, and get stronger. The major risk is when an animal is sick or debilitated for some reason--the stress can add to their illness and reduce their immune function.

Remember there are some folks who actually assist-feed hatchlings of other species for up to a year before the animals start feeding on their own--I recall an article in Reptiles magazine about someone raising taipans that way. Assist-feeding CAN be carried out for as long as it takes to get a baby going, and the vast majority of the time, if there isn't anything actually WRONG with them, they will go on to eat on their own. Don't let them get too thin, or to the point where they are debilitated, as that will make it harder for them to handle the assist or force-feeding.