Try feeding every 7 days, make sure the room is really dark, prescent the room for a about half an hour to an hour before, try leaving it in overnight as well, make sure the mouse isn't cool to the touch. I'm not sure if you've thought of these options already. Also try a smaller feeding size.

I have to do a lot of that with my female now. She's probably just over a year old, to a year and a half.. Not sure when she was born.

goodluck!