That tanks more than big enough then. It may just be that she has no idea or experience with offspring. Female rats eat the afterbirth (it's good for them) and some of them on the first go round, well...they just don't stop nibbling to put it bluntly.

Watch her around her next newborns or other newborns in your colony that come along. That should show whether this was a one time event or she's a rat you don't need in a breeding group.