You don't say what boat, how old etc. etc.. Many times the nuts come off and the bolts look fine but it the sealant/bedding compound has been leaking galvanic corrosion may still be eating away your bolts where you can't see them inside the keel stub.
Galvanic corrosion of keel bolts from the Ericson Owners Forums:
This is what my backing plates look like. They are original and the boat is a 1979 so I guess they used a high quality stainless from the factory.