If she's not molting, isn't getting picked on and isn't suffering from external parasites like mites or lice, it's possible that the bare spot on her front is caused by broodiness. Broody hens often lose the feathers on their lower breasts so that their bodies can be closer to the eggs they are trying to warm and incubate. It is called a "brood patch."
Silkies often go broody
Since this is a hormonal condition, it is possible she will lose or pluck feathers during broodiness whether or not she actually has any eggs. If she's been broody lately and you've eliminated other possible causes of feather loss, our guess would be that you're just seeing a brood patch. If you think she's ill or if she has other symptoms, you'll want to get her to an avian vet for a firm diagnosis and treatment options. We aren't vets and can't diagnose your bird.