Highly unlikely to be nutrition related as the others are bagging up ok. An adult cow can obtain sufficient protein from reasonable silage, and unless she is absolutely immaciated can produce enough milk "off her back".
But, as mentioned earlier, even in the (unlikely) event that it is nutrition related, the last thing you want to do is to greatly increase the protein in the later stages of pregnancy as it will all go into the calf and you'll then end up with calving problems. If you feel she needs more to eat then give her a bit of barley as well, and make sure she's not getting pushed out by the others.