What are the possible causes of illness when a female cow refuses to graze well even after deworming her?

She could be bloated, or have some other form of stomach upset that is causing her not to eat. In all honesty, the possibilities are endless, even more so when you do not provide any symptoms of her "illness" besides her not eating. Pretty well any illness can cause a cow to go off feed, so more symptom description would help. But please see your vet about this soon, as it could be very serious.