MIKE GEORGE says:
This does happen when an oilstone has had a lot of use.
The surface of the stone gets clogged with a mixture of tiny metal particles, muck, and old oil which has set hard.
To remedy the situation, give the stone a good soak in petrol, and scrub the surface with something like an old nailbrush.
You will be surprised at the amount of muck that comes out!
After the stone has dried, re-oil it with thin oil. Then you’re back in business.