If the joeys are close to being out, or just newly out, sometimes the mom will not leave the pouch much at all. This is where dad comes in to help bring her food and make sure that she is still eating. Since he's not around, she will need you to help her even more. Try to make sure she is hydrated, you can pinch a little bit of the skin on the back of her neck to see if it makes a "tent." If the skin does not go quickly back down, then she needs more fluids. If you are able to get a hold of something like pedialyte, you can give her that. I'm not sure what it would be called where you are, but it's a drink with electrolytes and minerals in it that you give to dehydrated babies. You can also try to encourage her to drink some apple juice, or flavor some water with honey to get her some fluids. It is possible that she is just lacking in nutrients from where she had been on such a poor diet before you got her, and nursing a joey is taking a lot out of her. Do your best to encourage her to eat and drink, and hopefully that will help perk her up.