
Are you feeding her four small meals a day? Many bitches not only can't cope with large meals but can't face a bowl full.
Also they may refuse food now and be happy to eat an hour later when things shift around and they are more comfortable.
Offer her the food four times a day and pick it up if she doesn't want it and offer again later. they will eat what they can and the pups will take what they need out of the bitch regardless.
Also how much are you expecting her to eat.
At the end of pregnancy mien are getting about 50% more than normal, but of a higher quality food than their normal rations, so you may be expecting her to eat a lot more than normal which she won't need to be, especially if you were over generous earlier in pregnancy.
I don't' increase the amount (though I switch to puppy food at 4 weeks in whelp) until they are 6 weeks in whelp.