This animal is not necessarily underweight or too small number is one thing overall look is another, if it's a male it would be perfectly in range, and if it's a female well they don't all grow at the same rate, I have had female barely weighing 900+ grams at 3 years.
Right now offer the same type prey previously fed (mice) once a week, 2 adult mice will be more than necessary and a good routine of once a week is more than enough as well.
Once she feeds consistantly for you is when you can attempt to switch (it's easier to switch a well established snakes that feeds for you than trying to switch one right of the bat). Switching should be done gradually, from live mice to f/t mice or from live mice to live rats and than the final step. Going from live mice to f/t rats can be a lot more complicated and more likely to lead to refusal.
Having this BP sexed will also help when it comes to feeding. For an adult male you are looking at feeding 2 to 3 mice a week depending on their size or a small rat (about 60 grams), and for a female you will never have to feed anything larger than a medium rat (but you are not there yet).
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