Start sanding too much and you'll take the sound right out of the box. Or at the very least, you'll change the sound dramatically. Once you start sanding wood off, you change the shape of the lid and that changes the sound.

If you can find some of the half moon shaped blue chalk that is out there, I have found it to be very good on every box call I've tried it on. NEVER use chalkboard chalk on a box call.

I suspect you just chalked her up a little too heavy and that a light sanding will get you back to where you need to be.