Riding Mower Won't Start?

I tried starting my riding mower and it would not start. It sounded like the starter, so I replaced the starter. It started right up, but the next time I went out to mow the grass the mower would not start. I replaced the battery, now all I get is a clicking sound and then a buzzing sound, again sounding like the starter.

So I replaced the new starter with another new starter (not rebuilt) again it started right up. I again cut the grass, and the next morning being just curious I tried to start my mower and got nothing but clicking. Help. It is a Sear's 20hp Craftsman 42 inches.

By Jim H. from Foley, AL