It doesn't sound like your female is too fat -- you might want to try feeding her 5 or 6 mice once a week instead of every couple of days -- that will give her body some time off from digesting.
I have a tried and true method (for me) for getting balls switched from live mice to rats -- it's always worked for me but as always every snake is different and results may vary.
First you get her used to feeding on the multiple mice -- throw one in she grabs it and eats it -- throw another in she grabs it and eats it, etc. until she's feeding on 5 or 6 mice.
The next week throw in the first mouse -- grabs and eats -- throw in another mouse -- she grabs and eats -- throw in a rat pup same size and color as the mice - 9 times out of 10 she'll be in feeding mode and won't even realize it's a rat not a mouse.
Try that for a few weeks and every week replace a mouse with a rat pup until you are just throwing in one mouse and a bunch of rat pups -- eventually you can wean her off the mice entirely and start bumping up the size of the rat.
Good luck!
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Tosha
JET Pythons
Toshas Blog
