Try heating it just a touch more before offering it. May be that she's having trouble "finding" it once she's killed it. That's worked for a couple I've had doing that.
Or, could just be a weird temporary quirk of hers that she should eventually outgrow. I've had others do that too. Balls can be real brats when they want to be.
I'm rehabbing a baby for a pal that's doing this, constricts and kills but does not eat anything larger than pinkies. No idea why he does it, but I've figured out that dropping in another prey item after he's left the first triggers him to constrict and eat, and eat much larger prey items. Eventually he'll outgrow this silly game.
As long as nothing has changed in her environment that's triggered this, just keep offering every week and wait her out.
ETA - helps to have a trash can snake ready to take the rats your persnickety ball doesn't want.