First litter could have a little to do with that, but something ain't right.
Either she's not producing enough milk or there's not enough nutrition going around.
If the "mouse blend" is primarily seeds, I'd say find something else. Blech.
Seeds are fine as treats occasionally, but not as a sole diet. There's no nutrition in there, just fat. And if it says "enriched", thats a crock too. They can't inject the vitamins in the seed cases, they just spray it on the outside which is what animals don't eat.
Find yourself a good quality lab block. Petsmart carries Mazuri which is good or maybe find a local feed mill that can make it like here. I buy my feed in 50lb bags and I've had good results with it.