When I started casting lead balls around that size for my 3/4 scale gun, I had the same results. If you cut into one of the light balls with a hacksaw, you should find a void somewhere between the sprue and half way down the ball. Making sure the mould is good and hot and having enough sprue capacity to feed the cooling ball should cure the problem. I usually heat my mould by casting balls in it until I start getting good casts, returning the bad casts to the melting pot. A large capacity sprue is very inportant in large capacity molds.