76-77 are winter breeding temps so that is why some are refusing.

I find finicky eaters do better in warmer ambient temps and smaller tubs/enclosures.

I recently received an adult female and I was told she was already fasting and refusing, after 6 days in my 82 degree ambient quarantine setup she nailed a rat first attempt.

I would also alternate mice to rats (live). Some will only do mice and not rats. At that size they really shouldn't be fasting unless they are cool or insecure/stressed.