Hmmm,
I take it you tried not offering food for a couple of weeks then offering a small rat?
My normal male is a fussy little beggar on occasion and will refuse his rat but will take a couple of mice. Since that doesn't fill him up however he will eventually take the rat (though it can take a month of mice feedings ) and be fine for another couple of months.
I will feed skinny hatchlings every three days on occasion but I'm happier with slower growth and a 5 day schedule - and anything over 500g I stick to a 7 day schedule as I think having a set feeding timeline actually helps them eat better.
Plus if they know when feeding day is it makes it a little less stressful for me to stick my paw into the tank on the other days.
dr del